The forums are littered with this question with only circumstantial answers. Some mention a setting that is no longer in the path they denote while others proclaim it to be your OS theme. This seems to be based on people running Win/Mac or *nix full [read: popular] DE’s. Despite that the number of questions should still indicate this does not work as intended or satisfactorily for many people and should be changed/revised to allow a user defined color.
Given I do not run a DE this answer is largely meaningless / useless. There must be a way to define the color that does not assume various system features exist much less require you to change your whole system theme / UI for one setting?!
It seems rather insane to allow users to define colors that for whatever reason might oppose or clash with a system theme to then allow the system theme to define/override the color theme set for the application.
At present the only cheese workaround is to install various GTK themes and launch with GTK_THEME=“something”.