Add custom/helper css classes - fix

Author: elibolonur

Diff for: src/docs/scss/app.scss

@@ -188,9 +188,9 @@ footer {
 }
 
 .bg-white{
-  background: #fff;
+  background: white;
 }
 
 .bg-black {
-  background: yellow;
+  background: black;
 }