Theme change not applied to README.md preview

### Describe the bug
Hey, I found a minor bug in the UI when changing themes from a page with `README.md`/`PREADME.md`.
The new theme will be applied everywhere except in the README preview.
Simply reloading the page will fix the problem.

It's really a minor bug. I don't really expect it to be fixed, but I wanted you to be aware of it :)

### To Reproduce
To see it, go to a page with `README.md`, then change the theme.

### Expected behavior
Apply the theme to the preview.

### Screenshots

### Server details (if you're NOT using docker/podman)
* run copyparty with `--version` and grab the last 3 lines (they start with `copyparty`, `CPython`, `sqlite`) and paste them below this line:
copyparty v1.19.23 "usernames" (2025-12-17)
CPython v3.13.9 on Linux64 [GCC 14.3.0]
sqlite 3.50.4*1 | jinja 3.1.6 | pyftpd 2.0.1 | tftp (None)