Add background color for tabs (#1170)

* Add background color for tabs

* Use BackstageTheme

* Use paper instead

* Use standard theme

packages/core/src/layout/HeaderTabs/index.tsx

@@ -18,12 +18,12 @@
 // This is just a temporary solution to implementing tabs for now
 
 import React from 'react';
-
 import { makeStyles, Tabs, Tab } from '@material-ui/core';
 
 const useStyles = makeStyles(theme => ({
   tabsWrapper: {
     gridArea: 'pageSubheader',
+    backgroundColor: theme.palette.background.paper,
   },
   defaultTab: {
     padding: theme.spacing(3, 3),