WIP: feat: add Dark mode

[tsonge]

• I've checked that this isn't a new swag opportunity proposal.
• I've checked that this isn't a duplicate pull request.

Fixes #106

Here is my attempt at adding a dark mode toggle feature. Open to feedback!

How it looks right now:

TODOs:

• Implement a nice dark mode toggle button.
• Set text to be dark in Selectr input drop down (tags filter input)
• Find out how to style iframe upscribe newsletter when CORS is disabled
• Use localStorage to remember user preference accross site visits
• switch colors of 'check it out' blue button when dark mode (improve contrast)

[tsonge]

@aslafy-z @vikaspotluri123 What do you guys think? I've done most of the things I wanted to do.

I could not manage to style the font color in the upscribe newsletter section since it is within an <iframe> element, with CORS disabled. Do you know how to fix that?