[Frontend] IN-PROGRESS Major restructuring of /platform/commonUI/ to add themes structure

open #95
New themes bundle files, .scss and directories;
Moved constants definitions (mainly colors) from commonUI/.../_constants
to individual themes _constants files;
tweaks to mixins files;
changed pathways to font and images in commonUI/.../_constants;

platform/commonUI/general/res/sass/_main.scss

@@ -21,7 +21,6 @@
  *****************************************************************************/
 @import "effects";
 @import "global";
-@import "fonts";
 @import "about";
 @import "text";
 @import "icons";