fix(button): fixed wrong md-icon selector and changed raised icon color to hue-900

Fixes #6944

  Closes #6945

Author: devversion

src/components/button/button-theme.scss

@@ -73,8 +73,8 @@ a.md-button.md-THEME_NAME-theme,
     color: '{{background-900}}';
     background-color: '{{background-50}}';
     &:not([disabled]) {
-      .md-icon {
-        color: '{{background-contrast}}';
+      md-icon {
+        color: '{{background-900}}';
       }
       &:hover {
         background-color: '{{background-50}}';