Admin backened is themed using bootstrap based css themes. 16 Themes are already provided with basic installation. Admin themes are located in root_dir > phpix-imports > adminui > abc.css where abc.css is the bootstrap based theme for admin. If there are dependent files like png or font, they can be kept in same folder. The only important thing to note is, that, there should be only 1 file for theme, if there are more files then please create a folder and keep extra files in that folder.

Now, you may want to see your adminui theme in action. Login to admin and goto options tab. You should see your theme listed in theme dropdown there. When giving name to a adminui theme, use small letters, numbers, hyphen, dots and underscore.

That's it friends. End of this story!