Verb (i.e. replacement for "clear") or adjective for "Blank"?
Sorry, this is a skin rather than a extension
I would say it's an adjective.
It could also be a noun :D But adjective is the best interpretation. It means it is a skin with no or very few styles that can be used as basis for customization.
That is indeed the case. The skin shows nothing, so it is a blank skin, with blank as an adjective.
Let us know if it's clear enough. https://translatewiki.net/w/i.php?title=MediaWiki%3ACustompage-skin-desc%2Fqqq&type=revision&diff=6922814&oldid=6826579