Revert "remove unneeded typecast"

This reverts commit 3975d93.

code/ui/blocks/src/components/Source.tsx

@@ -1,23 +1,27 @@
 import type { ComponentProps, FunctionComponent } from 'react';
-import { ThemeProvider, convert, ignoreSsrWarning, styled, themes } from '@storybook/theming';
-
 import React from 'react';
+import { styled, ThemeProvider, convert, themes, ignoreSsrWarning } from '@storybook/theming';
 import { SyntaxHighlighter } from '@storybook/components';
+
 import { EmptyBlock } from './EmptyBlock';
 
-const StyledSyntaxHighlighter = styled(SyntaxHighlighter)(({ theme }) => ({
-  // DocBlocks-specific styling and overrides
-  fontSize: `${theme.typography.size.s2 - 1}px`,
-  lineHeight: '19px',
-  margin: '25px 0 40px',
-  borderRadius: theme.appBorderRadius,
-  boxShadow:
-    theme.base === 'light' ? 'rgba(0, 0, 0, 0.10) 0 1px 3px 0' : 'rgba(0, 0, 0, 0.20) 0 2px 5px 0',
-  'pre.prismjs': {
-    padding: 20,
-    background: 'inherit',
-  },
-}));
+const StyledSyntaxHighlighter: typeof SyntaxHighlighter = styled(SyntaxHighlighter)(
+  ({ theme }) => ({
+    // DocBlocks-specific styling and overrides
+    fontSize: `${theme.typography.size.s2 - 1}px`,
+    lineHeight: '19px',
+    margin: '25px 0 40px',
+    borderRadius: theme.appBorderRadius,
+    boxShadow:
+      theme.base === 'light'
+        ? 'rgba(0, 0, 0, 0.10) 0 1px 3px 0'
+        : 'rgba(0, 0, 0, 0.20) 0 2px 5px 0',
+    'pre.prismjs': {
+      padding: 20,
+      background: 'inherit',
+    },
+  })
+);
 
 export enum SourceError {
   NO_STORY = 'There\u2019s no story here.',