To install a theme, download the theme, unzip it in the "themes" directory of your Ubooquity installation. Restart Ubooquity and go in the "General" settings section, you will see your new theme in the drop-down list. Select it, apply your settings change. That's it.
If you have already used Ubooquity, your browser might keep old versions of the theme files in its cache. If you notice wrong icons or colors, force the refresh of the page (Shift+F5 in most browsers).
To create a new theme, go to the "General" settings section and click "Create new theme". Once you have chosen the theme name, a directory with the same name will be created in the "theme" directory of your Ubooquity installation. It contains all the default theme files. Your theme is now ready to be edited. Modify these files as you like to change the way Ubooquity pages look.
To see your modifications, select your new theme in the "General" settings section and restart the server. Don't forget to force the refresh of the page after each modification to bypass the cache of your browser (Ctrl+F5 in most browsers).