Simle change of class by user/frontend input

I want to white label my application, by giving different users different colors of header, siderbar and buttons.   By using the html widget containing the overruling style works fine. (e.g. .btn-warning{background-color:Red !important}. But how can I set/select background-color dynamically via frontend ? (e.g. via a user settings form where I can enter  background color: Hex value…...)   -I know the are complete modules available (maui), but this is way to much, I only want to set about 20 values.
1 answers

Add 20 css variables to your project, and make use of the Whitelabel helper widget in marketplace.