In this version of FDT the FDT SWF Application launcher can handle CSS files:
The launcher starts the compilation of a CSS file into a SWF.

To create a launcher for a CSS file just right click the file in the FDT Explorer and choose Debug As/FDT SWF Application. By default the new created launcher does only invoke the compilation of the CSS into the SWF but it does not launch the final SWF, nor it starts the debugger.

For convenience the launcher WebApp-Dark.css can be added to the Launcher Chain of the web application launcher WebApp-WebApp:
The position in the Launcher Chain guarantees that the CSS file is compiled before the web application SWF is created and launched.