A desktop GUI in Python? Or a GUI in a web app? If you mean the former, have you tried wxPython? It comes with a huge interactive library of samples, where you can see the code and the resulting interface just by switching tabs.

There are so many languages that it can be real hard for beginners to decide what to do. I use Ubuntu and ChromeOS. Ubuntu recommends Python and GTK whereas, if I am not wrong, ChromeOS extension and webapps are primarily written in Javascript. I am learning programming currently using Codecademy HTML and CSS tutorials and have to decide what to do next. In order to keep myself focused I want to write a clipboard app. I am not sure what to choose Python or JavaScript.

The latter. I'll check that out, thanks.

I built a basic app with web.py that I'm trying to make pretty. I'm thinking I'll need to add some CSS, but I just have no idea where to start.

Have you seen the Python GTK+ 3 Tutorial[0]? I'm still pretty new to Python and it was a tremendous help.

[0]: http://python-gtk-3-tutorial.readthedocs.org/en/latest/

edit: Sorry, must've missed "web apps" when I first read your comment. The Django overview and tutorial were helpful to me as well.

See if any of these work for you:

https://www.google.com/search?q=flask+tutorial