Stopwatch and timer buttons - use standard CSS styles

This is more efficient and makes theming easier.

https://bugzilla.gnome.org/show_bug.cgi?id=706381
css: use defined colors from Adwaita

Hard coding colors in the Clocks css breaks the dark Adwaita
variant, and is generally Not Good.

https://bugzilla.gnome.org/show_bug.cgi?id=706393