[openstreetmap/openstreetmap-website] Add light/dark mode preference (Issue #5324)

[Will]

### Problem

Browsers do not let you set a per-site dark mode preference. Some users do not want to use the new dark mode.

### Description

A toggle similar to the Wikipedia appearance (see below). It should probably be accessible for users who are not logged in, as well.

### Screenshots

![image of Wikipedia's "appearance" menu](https://github.com/user-attachments/assets/9621932c-375e-4178-aec8-409e1e7b84c7)



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