#### Bug, feature request, or proposal: `mat-button-toggle-theme` mixin defines but does not use `$background` variable #### What is the current behavior? As result it uses `$mat-grey` palette for background colors #### Source code: [_button-toggle-theme.scss](https://github.com/angular/material2/blob/master/src/lib/button-toggle/_button-toggle-theme.scss)