Make right panel a dark-panel instead of every descendant.

Also move bg color central.

src/components/structures/RightPanel.js

@@ -213,6 +213,7 @@ export default class RightPanel extends React.Component {
         const classes = classNames("mx_RightPanel", "mx_fadable", {
             "collapsed": this.props.collapsed,
             "mx_fadable_faded": this.props.disabled,
+            "dark-panel": true
         });
 
         return (