Thanks for contacting us regarding this issue.
It depends on how the assembly was built and whether these CSS files have been compiled with the assembly as embedded resources or they are being used only as linked resources (which stay out of the assembly, the assembly manifest only contains paths to the resources themselves). If it is the first case, it is highly probable that you should find them in the root directory which you select during Project Generation for this assembly. However, if they are used only as linked resources, it might be possible that we miss copying them to the folder of the generated project. I suppose that this is your case. We will investigate further what can we do about this issue.
Thanks again for pointing us out this problem. Please, do not hesitate to contact us if you stumble upon any other issue. Happy decompiling!