Allow users to switch between dark and light mode

[ByteOtter]

• [x] Create a button/switch in the upper right corner of the main nav bar
• [x] Create a separate .css file for darkmode
• [x] Edit layout.html to allow switching stylesheets

[ByteOtter]

Implemented without the use of a sparate .css file

[ByteOtter]

fixed with creating two sets of CSS color variables. Nightmode now avaliable.