file: create styles.js and add @mui/styles within makeStyles

src/styles.js

@@ -0,0 +1,15 @@
+import { makeStyles } from '@mui/styles';
+
+export default makeStyles(() => ({
+    root: {
+        display: 'flex',
+        height: '100%',
+    },
+    content: {
+        flexGrow: 1,
+        padding: '6em 2em 2em',
+    },
+    toolkit: {
+        height: '70px',
+    },
+}));