VPN-6064 - detect dark mode

[mcleinman]

Description

I tested this by adding some code to run this when loading themes. I created another ticket to later add code to detect changes immediately.

Reference

VPN-6064

Checklist

• My code follows the style guidelines for this project
• I have not added any packages that contain high risk or unknown licenses (GPL, LGPL, MPL, etc. consult with DevOps if in question)
• I have performed a self review of my own code
• I have commented my code PARTICULARLY in hard to understand areas
• I have added thorough tests where needed