Add white colour for button background

Former-commit-id: 04810a7fc35049097c505c2dce4656ff762eb942
Former-commit-id: 48eeb2e7f1fd4bbba04f6a9e34740b2e14bcf6cc

src/elements/button.less

@@ -929,6 +929,12 @@
        Colors
 --------------------*/
 
+/*--- White ---*/
+.ui.white.buttons .button,
+.ui.white.button {
+  background-color: #FFFFFF;
+}
+
 /*--- Black ---*/
 .ui.black.buttons .button,
 .ui.black.button {
@@ -1173,4 +1179,4 @@
   -moz-border-radius: 0px 0px 0.2em 0.2em;
   -webkit-border-radius: 0px 0px 0.2em 0.2em;
   border-radius: 0px 0px 0.2em 0.2em;
-}
+}