Move mixin-based colors to variables

[saitho]

Pull Requests

Please accompany all pull requests with the following (where appropriate):

• [ ] unit tests (we use Jasmine 2.x.x)
• [ ] JSFiddle (or an equivalent such as CodePen, Plunker, etc) or screenshot/GIF with new feature or bug-fix
• [x] Link to original Github issue (if this is a bug-fix) https://github.com/seiyria/bootstrap-slider/issues/971
• [ ] documentation updates to README file
• [ ] examples within /tpl/index.tpl (for new options being added)
• [ ] Passes CI-server checks (runs automated tests, JS source-code linting, etc..). To run these on your local machine, type grunt test in your Terminal within the bootstrap-slider repository directory

Makes mixin-based colors overridable in variables, so layout can be adapted more easily to custom layout that uses CSS variables.