Themes/styles are out of hand

As of now, Rungeon uses 6 different themes (not including the default theme). Certain elements are given colors and styling specific to themes and it creates problems. For example, when implementing new pages, I find myself switching between themes (or working in one that I wasn't the day before due to testing). This makes it difficult when dealing with CSS and colors for obvious reasons. Not to mention, the 404 page has its own theme and styling, which is very different from the game and what I'm currently working on on my fork.

This also goes with the JS side of things. It took me a literal 10-15mins to find and fix the few broken links JS was manipulating in the `public/main.js` file. I can only imagine how long it'll take to create a new stage (for both myself and for new contributors). This codebase needs a complete makeover. Rungeon's future depends on it.

## Proposal

- Stages should be organized in a manner that is both easy to read, write, and manipulate for both contributors and Rungeon. (I'm thinking more along the lines of JS objects or JSON to tackle this)
- Better documentation/comments. __Very Important__.
- Halt on all work on themes until a set style has been established and #49 is patched. We need a consistent development area.
- Resolve CSS element stylings. For example, `...` are for links. Why are they styled as if they are buttons?

These are a few things that stood out, but there are many more opportunities for improvement.

## See Also
- #121
- #117
- #49
- #114
- #134