Light/dark-mode dropdown isn't visible under narrow displays

[chalin]

Sigh, this seems like it might be a tricky z-index issue.

To reproduce:

• Visit, say, https://www.docsy.dev/docs/
• Ensure that your display / view is wide enough.
• Click on the light/dark-mode menu
• You should see the dropdown options
• Narrow the view to less than 992px
• You should see the dropdown options disappear
  • If you disable the relative positioning of .dropdown, the menu options reappear

I'll investigate further as soon as I can.