Refactor styles to use more variables

This pull request changes the following:

- Hex color variables are defined in `colors.less`
- `syntax-variables.less` depends on `colors.less` 
- More syntax variables, that are now required by Atom, are added to `syntax-variables.less`.
- `index.less` depends on `syntax-variables.less` (and `colors.less` implicitly)

The goal of these changes is more accurate theming and more extensible stylesheets.  Suggestions are very welcome.