message: Update Percentage.tsx
  useTheme() on line 32 returns null so I tested this change to the import on my local copy, which gets the sonar plugin working. Other plugins seem to be importing from '@material-ui/core', e.g.
diff: import { BackstageTheme } from '@backstage/theme';
  import { makeStyles } from '@material-ui/core/styles';
  -import { useTheme } from '@material-ui/styles';
  +import { useTheme } from '@material-ui/core';
  import { Circle } from 'rc-progress';
  import React from 'react';
  