An internet search for
linux mint customise selection color produces this as the first hit:
https://forums.linuxmint.com/viewtopic.php?t=148972
which suggests a package called
gtk-theme-config.
Have you tried that?
Of course, this is dependent on you having a GTK-based desktop environment (Cinnamon, MATE, Xfce), not a QT one (KDE).