Your might want to study
Common tweaks to a REAPER theme sticky in the Color Themes forum...especially
Tweak: Changing colors and fonts section.
The .ReaperTheme file mentioned there has all the theme colors and fonts. They are text files, so you could even copy & replace sections you'd want from one to the other. But without knowing what you were copying, that could come with unwanted surprises. Whatever you do, remember backup copies and name your edited versions so you know what is what.