Correct the import of PageTheme to use relative path

packages/core/src/layout/Header/Waves.test.tsx

@@ -16,8 +16,8 @@
 
 import React from 'react';
 import { render } from '@testing-library/react';
-import Waves from './Waves';
 import { pageTheme } from '../Page/PageThemeProvider';
+import Waves from './Waves';
 
 describe('<Waves/>', () => {
   it('should render svg', () => {

packages/core/src/layout/Header/Waves.tsx

@@ -16,7 +16,7 @@
 
 import React, { FC } from 'react';
 import { makeStyles } from '@material-ui/core';
-import { PageTheme } from '@backstage/core';
+import { PageTheme } from '../Page';
 
 const useStyles = makeStyles<PageTheme>({
   wave: {