How to download all skins in Studio into SDK

But my custom skin is already derived from theme_support?

Then you already started with a copy of the theme_support files. 

If you're trying to do local Sass compilation, that's not (yet) supported via the Community SDK, and as you've discovered, it's not straightforward to obtain all of the core Sass files.  You can, however, find read-only copies in Studio.

So what do you recommend doing? Because I need to to the li set-responsive-options to do local Sass compilation through studio too so it will know which skin to serve right? Should I go in and manually copy all the contents of the read only files of theme_support?

Ah, sorry, I mis-spoke.  Of course we do support local Sass compilation via li serve-sass.  But the Community SDK code does not currently handle core theme code. 

Back to SuzieH 's original response, this could be considered a bug, and is worth reporting to Support.

In the meantime, you could use Parshant 's workaround, which I believe is to create a new custom skin derived from "theme_support".  This is effectively a copy of theme_support, and since it's custom, Community SDK should allow you to export it.  You could then rename your local copy to "theme_support" and see if that allows you to compile locally via li serve-sass.  Not sure if Parshant has actually done this exact thing.  I have not.

Another option is to manually copy files out of Studio.  You're right that that doesn't sound like much fun.

So create a skin in studio with its parent being theme_support, just call it theme_support,  don't change any files, and then do li export-studio-plugin and it should download what is in theme_support?

You couldn't call the child "theme_support" in Studio, but I believe that's what Parshant is suggesting, yes.