[Frontend] Time controller-related styling (CP > open117)

open #1515
open #117
Fixing selects;
Tweaks to constants and mixins to better
handle button dropshadowing across
themes;
(cherry picked from commit 3e34d06)

platform/commonUI/themes/snow/res/sass/_mixins.scss

@@ -1,5 +1,6 @@
 @mixin containerSubtle($bg: $colorBodyBg, $fg: $colorBodyFg) {
 	@include containerBase($bg, $fg);
+	@include boxShdw($shdwBtns);
 }
 
 @mixin btnSubtle($bg: $colorBtnBg, $bgHov: none, $fg: $colorBtnFg, $ic: $colorBtnIcon) {