Steps to replicate the issue (include links if applicable):

• Go to https://doc.wikimedia.org/oojs/master/ using either Firefox mobile or using Firefox devtools responsive design mode
• Click the menu button at the top of the screen

What happens?:
The menu is transparent, appears behind the page content, and clicking either "namespace" or "classes" does not produce any effect

What should have happened instead?:
The menu should appear above the page content, and should have an opaque background. Clicking either of the options should send the user to the relevant part of the site.

Other information (browser name/version, screenshots, etc.):

• Observed using Firefox mobile 103.2.0, and per conversation on IRC also when using Firefox devtools responsive design mode

Works as intended on Firefox desktop.