diff --git a/src/components/structures/MatrixChat.tsx b/src/components/structures/MatrixChat.tsx
index 9d3114c67c..8524938db9 100644
--- a/src/components/structures/MatrixChat.tsx
+++ b/src/components/structures/MatrixChat.tsx
@@ -50,7 +50,7 @@ import ThemeController from "../../settings/controllers/ThemeController";
 import { startAnyRegistrationFlow } from "../../Registration";
 import ResizeNotifier from "../../utils/ResizeNotifier";
 import AutoDiscoveryUtils from "../../utils/AutoDiscoveryUtils";
-import ThemeWatcher from "../../settings/watchers/ThemeWatcher";
+import ThemeWatcher, { ThemeWatcherEvent } from "../../settings/watchers/ThemeWatcher";
 import { FontWatcher } from "../../settings/watchers/FontWatcher";
 import { storeRoomAliasInCache } from "../../RoomAliasCache";
 import ToastStore from "../../stores/ToastStore";
@@ -131,6 +131,7 @@ import { ConfirmSessionLockTheftView } from "./auth/ConfirmSessionLockTheftView"
 import { LoginSplashView } from "./auth/LoginSplashView";
 import { cleanUpDraftsIfRequired } from "../../DraftCleaner";
 import { InitialCryptoSetupStore } from "../../stores/InitialCryptoSetupStore";
+import { setTheme } from "../../theme";
 
 // legacy export
 export { default as Views } from "../../Views";
@@ -463,6 +464,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
         this.themeWatcher = new ThemeWatcher();
         this.fontWatcher = new FontWatcher();
         this.themeWatcher.start();
+        this.themeWatcher.on(ThemeWatcherEvent.Change, setTheme);
         this.fontWatcher.start();
 
         initSentry(SdkConfig.get("sentry"));
@@ -495,6 +497,7 @@ export default class MatrixChat extends React.PureComponent<IProps, IState> {
     public componentWillUnmount(): void {
         Lifecycle.stopMatrixClient();
         dis.unregister(this.dispatcherRef);
+        this.themeWatcher?.off(ThemeWatcherEvent.Change, setTheme);
         this.themeWatcher?.stop();
         this.fontWatcher?.stop();
         UIStore.destroy();
diff --git a/src/components/views/dialogs/ModalWidgetDialog.tsx b/src/components/views/dialogs/ModalWidgetDialog.tsx
index 58c6c92a5e..1bc123c485 100644
--- a/src/components/views/dialogs/ModalWidgetDialog.tsx
+++ b/src/components/views/dialogs/ModalWidgetDialog.tsx
@@ -33,7 +33,7 @@ import { OwnProfileStore } from "../../../stores/OwnProfileStore";
 import { arrayFastClone } from "../../../utils/arrays";
 import { ElementWidget } from "../../../stores/widgets/StopGapWidget";
 import { ELEMENT_CLIENT_ID } from "../../../identifiers";
-import SettingsStore from "../../../settings/SettingsStore";
+import ThemeWatcher, { ThemeWatcherEvent } from "../../../settings/watchers/ThemeWatcher";
 
 interface IProps {
     widgetDefinition: IModalWidgetOpenRequestData;
@@ -54,6 +54,7 @@ export default class ModalWidgetDialog extends React.PureComponent<IProps, IStat
     private readonly widget: Widget;
     private readonly possibleButtons: ModalButtonID[];
     private appFrame: React.RefObject<HTMLIFrameElement> = React.createRef();
+    private readonly themeWatcher = new ThemeWatcher();
 
     public state: IState = {
         disabledButtonIds: (this.props.widgetDefinition.buttons || []).filter((b) => b.disabled).map((b) => b.id),
@@ -77,6 +78,8 @@ export default class ModalWidgetDialog extends React.PureComponent<IProps, IStat
     }
 
     public componentWillUnmount(): void {
+        this.themeWatcher.off(ThemeWatcherEvent.Change, this.onThemeChange);
+        this.themeWatcher.stop();
         if (!this.state.messaging) return;
         this.state.messaging.off("ready", this.onReady);
         this.state.messaging.off(`action:${WidgetApiFromWidgetAction.CloseModalWidget}`, this.onWidgetClose);
@@ -84,6 +87,10 @@ export default class ModalWidgetDialog extends React.PureComponent<IProps, IStat
     }
 
     private onReady = (): void => {
+        this.themeWatcher.start();
+        this.themeWatcher.on(ThemeWatcherEvent.Change, this.onThemeChange);
+        // Theme may have changed while messaging was starting
+        this.onThemeChange(this.themeWatcher.getEffectiveTheme());
         this.state.messaging?.sendWidgetConfig(this.props.widgetDefinition);
     };
 
@@ -94,6 +101,10 @@ export default class ModalWidgetDialog extends React.PureComponent<IProps, IStat
         this.state.messaging.on(`action:${WidgetApiFromWidgetAction.SetModalButtonEnabled}`, this.onButtonEnableToggle);
     };
 
+    private onThemeChange = (theme: string): void => {
+        this.state.messaging?.updateTheme({ name: theme });
+    };
+
     private onWidgetClose = (ev: CustomEvent<IModalWidgetCloseRequest>): void => {
         this.props.onFinished(true, ev.detail.data);
     };
@@ -127,7 +138,7 @@ export default class ModalWidgetDialog extends React.PureComponent<IProps, IStat
             userDisplayName: OwnProfileStore.instance.displayName ?? undefined,
             userHttpAvatarUrl: OwnProfileStore.instance.getHttpAvatarUrl() ?? undefined,
             clientId: ELEMENT_CLIENT_ID,
-            clientTheme: SettingsStore.getValue("theme"),
+            clientTheme: this.themeWatcher.getEffectiveTheme(),
             clientLanguage: getUserLanguage(),
             baseUrl: MatrixClientPeg.safeGet().baseUrl,
         });
diff --git a/src/settings/watchers/ThemeWatcher.ts b/src/settings/watchers/ThemeWatcher.ts
index 4d0c4a7171..77c8a391ae 100644
--- a/src/settings/watchers/ThemeWatcher.ts
+++ b/src/settings/watchers/ThemeWatcher.ts
@@ -8,16 +8,25 @@ Please see LICENSE files in the repository root for full details.
 */
 
 import { logger } from "matrix-js-sdk/src/logger";
+import { TypedEventEmitter } from "matrix-js-sdk/src/matrix";
 
 import SettingsStore from "../SettingsStore";
 import dis from "../../dispatcher/dispatcher";
 import { Action } from "../../dispatcher/actions";
 import ThemeController from "../controllers/ThemeController";
-import { findHighContrastTheme, setTheme } from "../../theme";
+import { findHighContrastTheme } from "../../theme";
 import { ActionPayload } from "../../dispatcher/payloads";
 import { SettingLevel } from "../SettingLevel";
 
-export default class ThemeWatcher {
+export enum ThemeWatcherEvent {
+    Change = "change",
+}
+
+interface ThemeWatcherEventHandlerMap {
+    [ThemeWatcherEvent.Change]: (theme: string) => void;
+}
+
+export default class ThemeWatcher extends TypedEventEmitter<ThemeWatcherEvent, ThemeWatcherEventHandlerMap> {
     private themeWatchRef?: string;
     private systemThemeWatchRef?: string;
     private dispatcherRef?: string;
@@ -29,6 +38,7 @@ export default class ThemeWatcher {
     private currentTheme: string;
 
     public constructor() {
+        super();
         // we have both here as each may either match or not match, so by having both
         // we can get the tristate of dark/light/unsupported
         this.preferDark = (<any>global).matchMedia("(prefers-color-scheme: dark)");
@@ -72,9 +82,7 @@ export default class ThemeWatcher {
     public recheck(forceTheme?: string): void {
         const oldTheme = this.currentTheme;
         this.currentTheme = forceTheme === undefined ? this.getEffectiveTheme() : forceTheme;
-        if (oldTheme !== this.currentTheme) {
-            setTheme(this.currentTheme);
-        }
+        if (oldTheme !== this.currentTheme) this.emit(ThemeWatcherEvent.Change, this.currentTheme);
     }
 
     public getEffectiveTheme(): string {
diff --git a/src/stores/widgets/StopGapWidget.ts b/src/stores/widgets/StopGapWidget.ts
index c17aa81aab..6f3d554c11 100644
--- a/src/stores/widgets/StopGapWidget.ts
+++ b/src/stores/widgets/StopGapWidget.ts
@@ -43,7 +43,6 @@ import { MatrixClientPeg } from "../../MatrixClientPeg";
 import { OwnProfileStore } from "../OwnProfileStore";
 import WidgetUtils from "../../utils/WidgetUtils";
 import { IntegrationManagers } from "../../integrations/IntegrationManagers";
-import SettingsStore from "../../settings/SettingsStore";
 import { WidgetType } from "../../widgets/WidgetType";
 import ActiveWidgetStore from "../ActiveWidgetStore";
 import { objectShallowClone } from "../../utils/objects";
@@ -52,7 +51,7 @@ import { Action } from "../../dispatcher/actions";
 import { ElementWidgetActions, IHangupCallApiRequest, IViewRoomApiRequest } from "./ElementWidgetActions";
 import { ModalWidgetStore } from "../ModalWidgetStore";
 import { IApp, isAppWidget } from "../WidgetStore";
-import ThemeWatcher from "../../settings/watchers/ThemeWatcher";
+import ThemeWatcher, { ThemeWatcherEvent } from "../../settings/watchers/ThemeWatcher";
 import { getCustomTheme } from "../../theme";
 import { ElementWidgetCapabilities } from "./ElementWidgetCapabilities";
 import { ELEMENT_CLIENT_ID } from "../../identifiers";
@@ -163,6 +162,7 @@ export class StopGapWidget extends EventEmitter {
     private viewedRoomId: string | null = null;
     private kind: WidgetKind;
     private readonly virtual: boolean;
+    private readonly themeWatcher = new ThemeWatcher();
     private readUpToMap: { [roomId: string]: string } = {}; // room ID to event ID
     // This promise will be called and needs to resolve before the widget will actually become sticky.
     private stickyPromise?: () => Promise<void>;
@@ -213,7 +213,7 @@ export class StopGapWidget extends EventEmitter {
             userDisplayName: OwnProfileStore.instance.displayName ?? undefined,
             userHttpAvatarUrl: OwnProfileStore.instance.getHttpAvatarUrl() ?? undefined,
             clientId: ELEMENT_CLIENT_ID,
-            clientTheme: SettingsStore.getValue("theme"),
+            clientTheme: this.themeWatcher.getEffectiveTheme(),
             clientLanguage: getUserLanguage(),
             deviceId: this.client.getDeviceId() ?? undefined,
             baseUrl: this.client.baseUrl,
@@ -245,6 +245,10 @@ export class StopGapWidget extends EventEmitter {
         return !!this.messaging;
     }
 
+    private onThemeChange = (theme: string): void => {
+        this.messaging?.updateTheme({ name: theme });
+    };
+
     private onOpenModal = async (ev: CustomEvent<IModalWidgetOpenRequest>): Promise<void> => {
         ev.preventDefault();
         if (ModalWidgetStore.instance.canOpenModalWidget()) {
@@ -288,9 +292,14 @@ export class StopGapWidget extends EventEmitter {
         this.messaging = new ClientWidgetApi(this.mockWidget, iframe, driver);
         this.messaging.on("preparing", () => this.emit("preparing"));
         this.messaging.on("error:preparing", (err: unknown) => this.emit("error:preparing", err));
-        this.messaging.on("ready", () => {
+        this.messaging.once("ready", () => {
             WidgetMessagingStore.instance.storeMessaging(this.mockWidget, this.roomId, this.messaging!);
             this.emit("ready");
+
+            this.themeWatcher.start();
+            this.themeWatcher.on(ThemeWatcherEvent.Change, this.onThemeChange);
+            // Theme may have changed while messaging was starting
+            this.onThemeChange(this.themeWatcher.getEffectiveTheme());
         });
         this.messaging.on("capabilitiesNotified", () => this.emit("capabilitiesNotified"));
         this.messaging.on(`action:${WidgetApiFromWidgetAction.OpenModalWidget}`, this.onOpenModal);
diff --git a/test/components/views/dialogs/ModalWidgetDialog-test.tsx b/test/components/views/dialogs/ModalWidgetDialog-test.tsx
new file mode 100644
index 0000000000..134aa46ad6
--- /dev/null
+++ b/test/components/views/dialogs/ModalWidgetDialog-test.tsx
@@ -0,0 +1,56 @@
+/*
+Copyright 2024 New Vector Ltd.
+
+SPDX-License-Identifier: AGPL-3.0-only OR GPL-3.0-only
+Please see LICENSE files in the repository root for full details.
+*/
+
+import { fireEvent, render } from "jest-matrix-react";
+import { ClientWidgetApi, MatrixWidgetType } from "matrix-widget-api";
+import React from "react";
+import { TooltipProvider } from "@vector-im/compound-web";
+import { mocked } from "jest-mock";
+import { findLast, last } from "lodash";
+
+import ModalWidgetDialog from "../../../../src/components/views/dialogs/ModalWidgetDialog";
+import { stubClient } from "../../../test-utils";
+import defaultDispatcher from "../../../../src/dispatcher/dispatcher";
+import { Action } from "../../../../src/dispatcher/actions";
+import SettingsStore from "../../../../src/settings/SettingsStore";
+
+jest.mock("matrix-widget-api", () => ({
+    ...jest.requireActual("matrix-widget-api"),
+    ClientWidgetApi: (jest.createMockFromModule("matrix-widget-api") as any).ClientWidgetApi,
+}));
+
+describe("ModalWidgetDialog", () => {
+    it("informs the widget of theme changes", () => {
+        stubClient();
+        let theme = "light";
+        const settingsSpy = jest
+            .spyOn(SettingsStore, "getValue")
+            .mockImplementation((name) => (name === "theme" ? theme : null));
+        try {
+            render(
+                <TooltipProvider>
+                    <ModalWidgetDialog
+                        widgetDefinition={{ type: MatrixWidgetType.Custom, url: "https://example.org" }}
+                        sourceWidgetId=""
+                        onFinished={() => {}}
+                    />
+                </TooltipProvider>,
+            );
+            // Indicate that the widget is loaded and ready
+            fireEvent.load(document.getElementsByTagName("iframe").item(0)!);
+            const messaging = mocked(last(mocked(ClientWidgetApi).mock.instances)!);
+            findLast(messaging.once.mock.calls, ([eventName]) => eventName === "ready")![1]();
+
+            // Now change the theme
+            theme = "dark";
+            defaultDispatcher.dispatch({ action: Action.RecheckTheme }, true);
+            expect(messaging.updateTheme).toHaveBeenLastCalledWith({ name: "dark" });
+        } finally {
+            settingsSpy.mockRestore();
+        }
+    });
+});
diff --git a/test/unit-tests/stores/widgets/StopGapWidget-test.ts b/test/unit-tests/stores/widgets/StopGapWidget-test.ts
index 61e96886b9..2422b176be 100644
--- a/test/unit-tests/stores/widgets/StopGapWidget-test.ts
+++ b/test/unit-tests/stores/widgets/StopGapWidget-test.ts
@@ -7,7 +7,7 @@ Please see LICENSE files in the repository root for full details.
 */
 
 import { mocked, MockedFunction, MockedObject } from "jest-mock";
-import { last } from "lodash";
+import { findLast, last } from "lodash";
 import {
     MatrixEvent,
     MatrixClient,
@@ -27,10 +27,15 @@ import { MatrixClientPeg } from "../../../../src/MatrixClientPeg";
 import { StopGapWidget } from "../../../../src/stores/widgets/StopGapWidget";
 import ActiveWidgetStore from "../../../../src/stores/ActiveWidgetStore";
 import SettingsStore from "../../../../src/settings/SettingsStore";
+import defaultDispatcher from "../../../../src/dispatcher/dispatcher";
+import { Action } from "../../../../src/dispatcher/actions";
 import { SdkContextClass } from "../../../../src/contexts/SDKContext";
 import { UPDATE_EVENT } from "../../../../src/stores/AsyncStore";
 
-jest.mock("matrix-widget-api/lib/ClientWidgetApi");
+jest.mock("matrix-widget-api", () => ({
+    ...jest.requireActual("matrix-widget-api"),
+    ClientWidgetApi: (jest.createMockFromModule("matrix-widget-api") as any).ClientWidgetApi,
+}));
 
 describe("StopGapWidget", () => {
     let client: MockedObject<MatrixClient>;
@@ -104,6 +109,24 @@ describe("StopGapWidget", () => {
         expect(messaging.feedStateUpdate).toHaveBeenCalledWith(event.getEffectiveEvent());
     });
 
+    it("informs widget of theme changes", () => {
+        let theme = "light";
+        const settingsSpy = jest
+            .spyOn(SettingsStore, "getValue")
+            .mockImplementation((name) => (name === "theme" ? theme : null));
+        try {
+            // Indicate that the widget is ready
+            findLast(messaging.once.mock.calls, ([eventName]) => eventName === "ready")![1]();
+
+            // Now change the theme
+            theme = "dark";
+            defaultDispatcher.dispatch({ action: Action.RecheckTheme }, true);
+            expect(messaging.updateTheme).toHaveBeenLastCalledWith({ name: "dark" });
+        } finally {
+            settingsSpy.mockRestore();
+        }
+    });
+
     describe("feed event", () => {
         let event1: MatrixEvent;
         let event2: MatrixEvent;