I would like to allow my webapp users to save custom CSS through a text field to modify the look of their GUI.
I guess there are some evil CSS hacks out there. What should I take care about?
One solution would be for you to provide the user options through some editor, allowing them to pick colour/size of various elements. Then generate the CSS yourself based on the options the user has provided.
This would prevent any nonstandard CSS, or CSS you do not wish to use to be saved.