As far as I know, the css that gets imported automatically, will have to have the same name as your widget, although during deployment all files in all folders are copied over.
There is no such thing as "compiling" your css, it imports.
But you could import the CSS upon demand using Vanilla JS or you could try Dojo in your define:
define(["xstyle!./path/to/file.css"], function(){
...
});
The best option is to use Webpack to bundle your (external) dependencies. Setting up the Webpack is a pain. I working on some templates to do so.
Or use SASS compiler to bundle the different SASS file into on [WidgetName].css with the help of Grunt