Replace a Sass variable with a CSS custom property - hover-transition (#10786)

res/css/_animations.pcss

@@ -34,6 +34,10 @@ limitations under the License.
     transition: opacity 300ms ease;
 }
 
+:root {
+    --hover-transition: 0.08s cubic-bezier(0.46, 0.03, 0.52, 0.96); /* quadratic */
+}
+
 @keyframes mx--anim-pulse {
     0% {
         opacity: 1;