Theme: Switch icon background to use bgColorContent

It's more semanticly correct then fcActive and looks the same or better on most themes Fixes jquery/download.jqueryui.com#335

Author: arschmitz

themes/base/theme.css

@@ -119,7 +119,7 @@ a.ui-button:active,
 .ui-icon-background,
 .ui-state-active .ui-icon-background {
 border: #003eff/*{borderColorActive}*/;
-background-color: #ffffff/*{fcActive}*/;
+background-color: #ffffff/*{bgColorContent}*/;
 }
 .ui-state-active a,
 .ui-state-active a:link,