"decrement-arrow" has wrong rotation

[slaubenberger]

Hi

First of all, thank you very much for providing such a nice widget library!

Now the small issue: the "decrement-arrow" has an the wrong rotation in the following css-files: adwaita.css adwaita-dark.css cupertino.css

I fixed it by simply removing the unnecessary "rotate"-command in the css. It would be nice if you could include this fix in your main branch and upcoming releases.

Regards, Stefan