Dark theme: Page flashes white on reload/change

**Describe the bug**

When dark theme is used, reloading or visiting another link causes a white flash due to theme style transitioning.

**To Reproduce**

Steps to reproduce the behavior:
1. Go to https://academicpages.github.io/
2. Set dark theme if not already using it due to system preference
3. Click on the links at the top bar (or anywhere else)
4. See issue (page will flash white before applying dark theme)

**Expected behavior**

Theme should be applied seamlessly on page change/reload.

**Screenshots**

https://github.com/user-attachments/assets/8cf6cd73-caba-41cc-bbc0-b83106337203

**Browser (please complete the following information):**
- OS: iOS (also tested on Linux)
- Browser: Firefox
- Version: 152.0.1 (aarch64)

**Additional context**

Fixed by #3647.