Certain themes like the community theme "Homosapien" have transparency in particular apps such as Gnome Terminal and Gnome System Monitor. Is this a bug?
Screenshot:
http://img408.imageshack.us/img408/5165/transparentj.png
Ubuntu – Why do some GTK+ themes have transparency
gtkthemes
Related Solutions
From your screenshot - perhaps just double check that you haven't got your opacity of popup windows slightly transparent (assuming you are using the default compositor)
On Ubuntu 17.10 Qt4 apps do inherit the global GTK theme, while Qt5 don't. Since libqt5libqgtk2 isn't available in the official Ubuntu repositories anymore, we have to use another workaround:
Install qt5-style-plugins from the Ubuntu repositories with sudo apt install qt5-style-plugins.
Then add export QT_QPA_PLATFORMTHEME=gtk2 to ~/.profile by echo "export QT_QPA_PLATFORMTHEME=gtk2" >> ~/.profile. This will theme all apps started as user.
If you prefer to apply the theming system-wide (for user and root apps), then use sudo sh -c "echo 'export QT_QPA_PLATFORMTHEME=gtk2' >> /etc/environment".
For Ubuntu 20.04+ you have to sudo apt install appmenu-gtk2-module, so that qt apps don't start with a delay.
I prefer this method over using kvantum, because with this method also root apps are themed properly and you don't have to adjust the theme in the kvantum manager, to match it your gtk theme.
Reboot and enjoy!
Best Answer
This is a feature of the murrine engine that powers some themes. You only see it on applications that have support for it, such as gnome-system-monitor, unless you add the rgba module to GTK+, which honestly I don't suggest you do unless you are willing to put up with the work required to find all the incompatible programs and add work arounds.