[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/themes/espresso/res/sass/_mixins.scss

@@ -0,0 +1,9 @@
+@function pullForward($c: $colorBodyBg, $p: 20%) {
+    // For dark interfaces, lighter things come forward - opposite for light interfaces
+    @return lighten($c, $p);
+}
+
+@function pushBack($c: $colorBodyBg, $p: 20%) {
+    // For dark interfaces, darker things move back - opposite for light interfaces
+    @return darken($c, $p);
+}