Hello,
I think that @Unicorn777 is speaking about “Appearance Preferences > Customize Theme > Colours” as this screen capture taken by @wolfman in the fourth post in this thread https://ubuntu-mate.community/t/re-creating-the-classical-ubuntu-orange-theme/3062/4
Sadly, this functionality is gone … no more available.
You need to patch manually the theme to change colors.
Kr