Simplify the process of adding new themes

[JSerwatka]

The way highlightjs handles a theme styles is a bit complicated.
I like how it is simplified in carbon.

I want to make themes customizable so that users can save their own themes. However, the current way of handling themes is not suitable and would require a lot of work to make it similar to carbon themes. I won't start working on it unless it's a highly requested feature.