As Leonardo said, it's possible. But basically because anything is possible when you have the source code, rather than it being a considered feature.

Tips:

1. Get useful information from the styles in your browser by enabling debug mode by changing the URL in your browser: 
   • not: .../web#vie..... etc
   • yes: .../web?debug=#vie... etc
   • This will prevent odoo from minifying the css files and you can see the name of the css files and browse them as they are in the source code.
2. If you right click and inspect element (chrome or firefox are good for this) you can see the css styles of a DOM Element in your Odoo pages.
3. Override the css using a custom module: I've never tried this, one of the below ought to work though.
   • You could create a module and ensure that your css files are loaded later than Odoo's and that will override any styles you define, but wont neccessarily remove all odoo formatting (might be more desireable). I expect this would cause an odd lag as each css files is loaded in-turn during debug mode. But normal mode when the css is minified, it shouldn't be noticeable.
   • You could try to find the xpath which includes the .css files inside a .xml document. Then in your own module use an xpath to replace the .css file includes (I don't have an example of this because I'm bad with xpaths, but they seem to be pretty powerful so it should be possible.). This might provide complete override of any css file which might be desireable (but would be harder to get right).
   • I would avoid trying to replace any actual source files.
4. As for changing the css files you're on your own. It would be nice if you could document your findings somewhere. This is something I might try and do in the future as well.

As I said at the start this isn't really an Odoo feature so there's no garauntee the css class names will stay the same over the course of new releases.  Which means you'll have to come up with some extensive test and/or really pay attention to commits which effect css files defined by Odoo. If their files don't change your files shouldn't need to either.