WordPress CSS Adder Plugin
This plugin is useful if you want to customize your theme easily without modify the theme files or the original CSS.
Is not meant to contain extensive CSS (only by the optimization point of view) but it works perfectly to add your personal touch to a website without spending time and money on editing original files.
Plus, it is the solution for the main problem with the theme versions: if you buy and customize a WordPress theme and a new version comes out, editing the main theme files you will not be able to easily update your theme to the newer version!
With this plugin you can forget this problem and keep separate the original theme files and your customization!