Please note that changes are visible within minutes. Errors you make here can disrupt the entire encyclopedia, so make sure you know what you are doing. Always check with the W3C CSS Validation Service before and after any changes.
Testing can be done on your personal skin.css. In Mozilla Firefox and Opera, you can also test style changes dynamically with the test styles bookmarklet from squarefree.com. It pops up a window for adding style rules, and updates the page as you type.