remove transparency for pop-up menus

[RedAuburn]

thank you so much for your work on Clapper, and the 0.6 release!
i'm loving it so far, my biggest issue is that with the transparency it's a bit hard to read the text in pop-up menus.

eg:

IMO it'd be good to make these menus opaque again.

[Rafostar]

We can tweak opacity and colour if necessary. Just note, that there is no colour that will be visible (distinguishable) on every colour. Currently it was done to be slightly lighter to be visible on black borders. OSD elements should have transparency through.

[nekohayo]

IMHO, the only way this could be readable while transparent would be by having a blur API:

• GNOME Shell issue and corresponding Mutter issue
• Wayland protocol proposal

[Rafostar]

Yes. I agree that blur would fix all this, but since we do not have it yet the best I can offer is to make it less transparent a very tiny bit.