Manjaro XFCE default theme matcha-gtk-theme should have all needed CSS selectors
$ grep "theme_selected_bg_color" /usr/share/themes/Matcha-sea/gtk-3.0/gtk.css
@define-color theme_selected_bg_color #2eb398;
Create a CSS snippet file ~/.config/gtk-3.0/gtk.css and add this to change highlight color:
@define-color theme_selected_bg_color #2eb398;
Change the color code to match your preferred theme
and log out or reboot system to reload theme