use opacity for muted clock elements

This allows us to use different colours for different timers,
which will be useful when we add a pacing timer.

plugin/notes/notes.html

@@ -124,7 +124,7 @@
 				}
 
 				.speaker-controls-time span.mute {
-					color: #bbb;
+					opacity: 0.3;
 				}
 
 				.speaker-controls-notes {