Gray text highlighting no longer works on my Debian/stretch system. While this is particularly annoying in Emacs, it's not a specific Emacs problem; gray background is also gone from other applications, including the web browser. Therefore I asked for help at Unix SO.
I do however have an Emacs-specific question: Are there ways of active-region highlighting other than gray background? I would like to try them out in order to narrow down the gray-background-is-gone bug.