This question already has an answer here:
I have this issue with some form fields in Firefox and using the
adwaita-dark theme. In some webpages where the field background is white, you cannot read the text because it is also white.
This question was edited in order to clarify that this kind of wording of the question might help people who are not familiar with what a GTK+ theme. I also thing that the wording will help to get more hits through search engines. If it's still considered a duplicate, I'll accept it.