In Illuminated Cloud I can see all of the Lightning Component and App bundles in my org, as well as all of the bundled source and metadata files that each contains.
However, when I try to SAVE (deploy) the .css file that's part of a component bundle, I get the following error popup.
As you can see, I am trying to save the "helloWorld.css" file in my "helloWorld" component bundle. I have been able to successfully save .cmp files, but not a .css file. To be honest, I'm not sure if I have ever been able to save a .css file in a Lightning bundle.