That's the Ribbon, and thanks to Microsoft, WB's can't skin it. Many of us use OldNewExplorer  http://www.softpedia.com/get/Tweak/System-Tweak/OldNewExplorer.shtml  to get rid of the ribbon so WB's can do what it does best. Note: The Action Center, Clock/Cal and systray hidden icon area also cannot be skinned by WB's.

I bought a key for the full version meanwhile. I'm even considering to make custom skins myself, but it seems SkinStudio, which I understand is the software to use for this, is not available separately anymore?

At least I only found it as part of the whole Object Desktop Suite? Can I still get Skin Studio alone somehow or is there an alternative that works with WB?

Skinstudio is bundled with Windowblinds. When you install WB SKS will install.

If you are a Object Desktop member (subscription for one year) SKS is available as a separate download on the "My Downloads" page

Ok, I checked the mail from Stardock Sales having purchase info and WB key - indeed it says I purchased WB and got SkinStudio added for free.

But SkinStudio does not appear in the list of "My Downloads", only WB. I even dl-ed and installed the whole WB package again since I thought it might be in there, but not in the trial version I had first. But it seems SkinStudio itself is nowhere on my computer, and selecting "edit style with SkinStudio" in WB only leads me to the product page.

Before I ask the support, is there something else I can do?

What I said......

I manually added it to your account.  Sorry for the confusion.