chore: revert default to single theme

src/node/markdown/markdown.ts

@@ -60,9 +60,7 @@ export const createMarkdownRenderer = async (
   base = '/',
   logger: Pick<Logger, 'warn'> = console
 ): Promise<MarkdownRenderer> => {
-  // TODO: cleanup
-  // const theme = options.theme ?? 'material-theme-palenight'
-  const theme = options.theme ?? { light: 'github-light', dark: 'github-dark' }
+  const theme = options.theme ?? 'material-theme-palenight'
   const hasSingleTheme = typeof theme === 'string' || 'name' in theme
 
   const md = MarkdownIt({