
Does not happen with adwaita

Do you mean these buttons here?

If they are "insensitive", the drawing section should be "backdrop-insensitive", not just "insensitive", so the code applied should be this one
Does transparency look like a bug?
No they are not insensitive. I could press that button and it would start the game. Did you mean this?
Ah OK, so I don't get why you think that this line should affect the transparency
https://github.com/ubuntu/yaru/blob/master/gtk/src/light/gtk-3.20/_drawing.scss#L329
This section is for "insensitive" buttons (not in backdrop). Normal buttons in backdrop are styled with this https://github.com/ubuntu/yaru/blob/67cd850a5d83d2d6340d7c85e57b59424b401d28/gtk/src/light/gtk-3.20/_drawing.scss#L384
Oh sorry. /selffacepalm
I'll update the OP
No problem, but my main question is "do you think this is a bug?". I can see it as a style choice, as long as it's consistent.
I never saw a transparent button before in the light theme. So I consider it to be not wanted. But I could be totally wrong :D
They might be all transparent, but nowhere there's a picture behind them to show it :D
The semitransparency doesn’t necessarily look wrong to me…