style: Don't round corners of window screenshot backgrounds

This is an unwanted side effect of commit aafa011f, override the inherited style to remove the border radius again.

Fixes #6168 (closed)

@snwh: I'm in doubt whether removing the border-radius is the right approach, or whether the bit that extended %osd_panel instead of only setting the background color should be reverted. It seems like %osd_panel could easily pull in other unwanted properties ...