Missing text highlighting Color when selecting it in Preview When I select text in PDF file in Preview it doesn't highlight properly.

This issue is reproducible after a restart. I'm on Mojave 10.14.1

I mean it's not about highlighting words or something, that works. But when I select text to copy it, it doesn't highlight although I can copy it and paste somewhere. Text is selectable, and it used to work like week ago, but now it doesn't. Any ideas?
If I understand your question..

The text highlight (select) color for Preview is set in your 

System Preferences > General



You can customize it by selecting Other then choose the color and how dark you want it by moving the slider.