Cache and performance
All is cached so the performance impact is neglectable. The rewritten CSS files are included in the CSS aggregation mechanism.
Code example
Code example for developers to add UI configurable CSS to their project.
How to add configurable CSS to your module, theme or site?
Just by suffixing default CSS values (plus unit) with a code comment in the css, scss or less file (7.x-2.0 version).
How to provide form elements on your settings page?
Settings can be added to your settings page the usual way.