added ability to make dark theme even darker with black backgrounds

I really like this theme in general, however, I really wanted the sidebar to have a black background and for the theme to be a little darker all around (It seems to be better on my eyes to have as much black as possible...and I personally feel like it looks much better, especially on a retina display). So I added the ability to have a black background in both the sidebar and in the syntax theme via a setting: `flatland_black_background`, which is set to either `true` or `false`. This way, I haven't changed any existing functionality/styles of the theme while still adding this feature for those who want a black background. I also updated some of the coloring of some components when using the black background because they seemed to work much better given the changes made.

I hope that I've adhered the standards of this project, I tried to study the coding style and make the changes in the same fashion...or at least as closely as possible.