Change the "text selection" or "highlight" color

[Aravisian]

Evince and Gedit both do not use the system theme to choose text highlight color. They both contain their own. I prefer to use Xed, in which you can use your own custom theme and colors (I have made several that are floating around the web as downloadable themes.)
You happened to pick two apps that wouldn't show the change.

I went through this not long ago with user @Frog on changing the same exact detail in Vivaldi browser. Let me see if I can find that thread and what we ultimately did...

EDIT: That thread is here:

How to change the high-lite background color

When I want to copy text on the board I can't see what I am coping due to the fact I use a dark theme background on the board (and every where else) and the high-lite back ground color is also black .... how can I change this color to something else .... thanks

We ultimately used Vivaldi Experimental to retheme Vivaldi. However, going to Settings > Appearance > Themes may be all you need to do - try a different theme.