theme: Darken hovered message-media-control button

Increases contrast between normal and hovered states in
message-media-control buttons. Previously there was very little
difference between the two states, making it hard to distinguish
whether the mouse cursor was over the button.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/855

data/theme/gnome-shell-sass/_common.scss

@@ -1158,7 +1158,7 @@ StScrollBar {
 
             &:last-child:ltr { padding-right: 18px; }
             &:last-child:rtl { padding-left: 18px; }
-            &:hover { color: $fg_color; }
+            &:hover { color: darken($fg_color, 10%) }
             &:insensitive { color: darken($fg_color,40%); }
           }