My Emacs version is
GNU Emacs 26.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.10).
region face looks like this.
Two things stand out:
regionface is unreadable.
DistantForegroundattribute seems to be set by GTK.
How can I disable this functionality, so that Emacs only uses the faces that I explicitly set through themes?