remove even more BackstageTheme usages

Signed-off-by: Fredrik Adelöw <freben@gmail.com>

plugins/code-coverage/src/components/CoverageHistoryChart/CoverageHistoryChart.tsx

@@ -15,7 +15,6 @@
  */
 
 import { useEntity } from '@backstage/plugin-catalog-react';
-import { BackstageTheme } from '@backstage/theme';
 import {
   Box,
   Card,
@@ -50,7 +49,7 @@ import { DateTime } from 'luxon';
 
 type Coverage = 'line' | 'branch';
 
-const useStyles = makeStyles<BackstageTheme>(theme => ({
+const useStyles = makeStyles(theme => ({
   trendDown: {
     color: theme.palette.status.warning,
   },