Python Desktop Application with the Browser as an interface?
There are plenty of excellent GUI tools for the way you want to do your GUI -- HTML, CSS, and Javascript.  If you don't know of any, ask in a separate question with the right tags.
The Python side in such an arrangement should have no GUI of its own, but just run a subclass of the Python's standard library's HTTP server, just serving the HTML, CSS, and JS files, and data via JSON on other URLs that the JS can reach with Ajax techniques, essentially implementing storage and business logi -- so it's far from obvious what "GUI tool" you could possibly want for it?!
Just develop the Python side on its own (e.g. with IDLE, Wingware, SPE, or whatever you like) and the HTML / CSS / Javascript separately, with its own "GUI tool".  All that Python will do with those files is statically serve them, after all.
You could be thinking of using some Python side templating, such as Mojo &c, but my recommendation is to avoid that: rather, go with the "thin server architecture" all the way, make the Python side a RESTful server of business logic and storage layers, and do all the GUI work in the browser instead.
I want to create an application that runs on the users computer, a stand-alone application, with installation and what-not, but I want the interface to be a browser, either internal and displayed as an OS window or external accessible using the browser (i.e. some http server).
The reason would be because I know a little about Python, but I think I can manage as long as I have some basic roots that I can use and manipulate, and those would be HTML, CSS, and Javascript.
I've yet to find a good GUI tool which I can use, and always abandon the idea after trying to mess around and eventually not getting anything.