Fix colour regressions (#11273)

* Fix color contrast of notice events

Fixes https://github.com/vector-im/element-web/issues/25808

* Restore alert color for highlighted events

Fixes https://github.com/vector-im/element-web/issues/25788

* Fix quote border for highlighted events

Fixes https://github.com/vector-im/element-web/issues/25811

* Fix media background mixing with event highlighted

Fixes https://github.com/vector-im/element-web/issues/25812

res/css/views/messages/_MNoticeBody.pcss

@@ -16,5 +16,5 @@ limitations under the License.
 
 .mx_MNoticeBody {
     white-space: pre-wrap;
-    opacity: 0.6;
+    color: $secondary-content;
 }

res/css/views/messages/_MediaBody.pcss

@@ -18,7 +18,7 @@ limitations under the License.
 /* have unique styles). */
 
 .mx_MediaBody {
-    background-color: $quinary-content;
+    background-color: $panels;
     border-radius: 12px;
     max-width: 243px; /* use max-width instead of width so it fits within right panels */