fix text color of tech-radar footer (#3245)

.changeset/smooth-hairs-breathe.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-tech-radar': patch
+---
+
+Make the footer color of the tech-radar work in both light and dark theme.

plugins/tech-radar/src/components/RadarFooter/RadarFooter.tsx

@@ -15,19 +15,19 @@
  */
 
 import React from 'react';
-import { makeStyles, Theme } from '@material-ui/core';
+import { makeStyles } from '@material-ui/core';
 
 export type Props = {
   x: number;
   y: number;
 };
 
-const useStyles = makeStyles<Theme>(() => ({
+const useStyles = makeStyles(theme => ({
   text: {
     pointerEvents: 'none',
     userSelect: 'none',
     fontSize: '10px',
-    fill: '#000',
+    fill: theme.palette.text.secondary,
   },
 }));