UserForms stylesheets are written in SASS, so to make changes you'll need to have Compass installed.
$ apt-get update
$ apt-get install ruby
$ gem install compassInstall homebrew. Then:
$ brew update
$ brew install ruby
$ gem install compassMake your changes to scss/UserForms.scss or scss/UserForms_cms.scss. Then navigate to the userforms folder and run:
$ compass compile