I would like to find an extension that manages custom product labels using only CSS for shapes, colour, text.
All the extensions that I have checked use images (jpgs, etc) to create the background image.
After more research:
IWD - Yes, is an option
Amasty - No, it does have an option to edit label with css but an image must be uploaded to use a label.
Yes, you are right. Our Custom Stock Status supports only images. But they are quite customizable, and the admin can add more if he needs.
Anyway, good luck to you in searching the solution you're looking for!