dev server dark mode toggle bug

[patrickjcasey]

Describe the bug
The light/dark mode toggle does not work when running the development server via cargo xtask site serve

To Reproduce

• cargo xtask site serve from the root of the repository
• attempt to toggle to dark mode

Expected behavior
I would expect the mode of the website to change when toggling between the different light/dark mode settings

Screenshots

Desktop (please complete the following information):

• Operating System: Ubuntu 22.04
• Hipcheck Version 3.9.1 (dev version only)
• Firefox 133.0.3

Additional context
This appears to only be an issue with the dev server, as https://hipcheck.mitre.org appears to be working as expected