docs: match css filename in dark-mode.md (#30021)

* Update dark-mode.md

Fix the unmatched css file name

* Update docs/tutorial/dark-mode.md

Co-authored-by: Jeremy Rose <[email protected]>

Co-authored-by: Jenux <[email protected]>
Co-authored-by: Jeremy Rose <[email protected]>

docs/tutorial/dark-mode.md

@@ -80,9 +80,9 @@ Starting with the `index.html` file:
 </html>
 ```
 
-And the `style.css` file:
+And the `styles.css` file:
 
-```css title='style.css'
+```css title='styles.css'
 @media (prefers-color-scheme: dark) {
   body { background: #333; color: white; }
 }