Additional variables for theming

[geoffrey-eisenbarth]

In my own project I've implemented the following variables and I was wondering if there's any interest in merging them in. I find that they help a lot with the "theme-ability" of missing.css.

1. --border-width
2. --interactive-fg
3. --box-shadow
4. --box-shadow-hover
5. --box-shadow-active
6. --box-shadow-disabled

Where the --box-shadow variables are for the different button, .\<button\> states.

Thoughts?