I have Ubuntu MATE 18.04 LTS installed. Previously I got this problem on MATE 17.10.
I like old-fashioned Human-Clearlooks theme, so I installed
But it has strange silver background color in window titlebar:
I have already reported bug 1721955 to LaunchPad, it confirmed by other user.
How can I debug the issue? Do you have any ideas?
How to get GTK2-based themes working again in GTK3-based Ubuntu?
Update 20180505: Ubuntu MATE 18.04 LTS is affected too. We have a discussion on ubuntu-mate.community.
Update 20190128: Ubuntu MATE 18.10 and upcoming 19.04 are affected too.