in Geany, press "tools" > Configuration Files > geany.css Ctrl + F ": image(-gtk-recolor(url("assets/scalable/small-checkbox-unchecked-symbolic.svg")), -gtk-recolor(url("assets/scalable/small-checkbox-unchecked-symbolic.symbolic.png")));" put #/ at the beginning. Put /# at the end. Ctrl + S Close, reopen. Checkboxes work now.
it becomes light mode when I do that
as suggested by the other person, upload your css file. or even a snippet of it
I've literally just installed geany, first ever time using it, installed the gtk theme and commented out that line and got checkboxes rather than red squres