How to change highlight color?

I like the built-in theme but there’s one issue:
When you highlight a variable name, it also highlights that name everywhere else in that file.
But that secondary highlight is so faint I can’t easily see it. How do I change the color/intensity of that secondary highlight?

See example, I highlighted “product” but the secondary highlight is very faint:

I like using as a GUI to change colors etc in my theme

This is probably what you’re looking for:

Open a theme to edit, you can find this setting in EDITOR > Editor