The author of the theme “Flat-Plat” has changed the name of his theme - it is now called “Materia-Theme”.
At the same time he now supports budgie-desktop 10.4 - hurray!
So stuff like transparency and the menu button and other budgie popovers are now correctly themed.
Since we cannot (easily) change userspace stuff like themes, you will have to make the tweak yourself. We have done the first part - materia-theme is now automatically installed for flat-plat users.
So for all flat-plat users - just go to budgie-settings - style and select one of the materia themes that has been installed.
You can now safely remove the package “flat-plat” i.e. sudo apt purge flat-plat
And if you want to know what all the fuss is about - you can try out materia-theme yourself - just open budgie-welcome - default settings and install the Materia theme (you have enabled backports haven’t you?!)