Add support for light/dark themes

[ryanjwessel]

It would be nice to be able to switch between the current Material Palenight-inspired theme and a light-theme.

The functionality would look similar to Dan Abramov's personal blog: Overreacted.

This effort would involve fixing global theming. The createGlobalStyle API from the styled-components library caused issues on my last attempt at integrating it, so I replaced it with a plain CSS file. This is problematic because I cannot maintain theme constants between CSS and JS files.

This Issue would involve one of the following:

1. Updating the styled-components implementation to handle global styles with Next.js's server-side rendering
2. Using a different approach