Light-to-dark mode transition on screen refresh

[csebianlander]

Describe the bug
If you're logged in and have set dark mode, then every time you refresh the screen (or, e.g., navigate to a new post or subsite) the screen will flash white and then re-transition to dark mode.

To Reproduce
Steps to reproduce the behavior:

1. Go to metastaging
2. Log in
3. Set dark mode
4. Refresh
5. Navigate to another page

Expected behavior
There should be no "transition" -- elements should be in dark mode upon initial page load.

Desktop (please complete the following information):

• OS: macOS 15.5
• Firefox 138.0.4

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.