Hello Jaime Weise,
Thank you for your feedback. I am glad you managed to find a solution for your problem.
As for .css files, all files within the Global folder of your theme are automatically included, as you wrote, so this seems to be fine.
If you really need this functionality, you can always create a custom control that will allow you to specify relative paths and look up those paths relative not to the current folder, but to the folder you want.