Adjust background colours of back button and font slider

Author: andybalaam

res/themes/light-high-contrast/css/_light-high-contrast.scss

@@ -6,6 +6,7 @@ $secondary-content: #5E6266;
 $tertiary-content: $secondary-content;
 $quaternary-content: $secondary-content;
 $quinary-content: $secondary-content;
+$roomlist-button-bg-color: rgba(141, 151, 165, 0.2);
 
 $username-variant1-color: #0A6ECA;
 $username-variant2-color: #AC3BA8;
@@ -97,3 +98,11 @@ $roomtopic-color: $secondary-content;
         color: $input-darker-fg-color !important;
     }
 }
+
+.mx_UserMenu_contextMenu .mx_UserMenu_contextMenu_header .mx_UserMenu_contextMenu_themeButton {
+    background-color: $roomlist-button-bg-color !important;
+}
+
+.mx_FontScalingPanel_fontSlider {
+    background-color: $roomlist-button-bg-color !important;
+}