Automatic color contrast checker?

I'm creating a theme and provide colors to certain elements. All goes well, except for some strange behavior. For example, when I define `ui.*.selectionBackground` to a certain color, this is ignored in drop-down menu's. But when I redefine that color to a bright one, it *does* appear.

The same is true for defining colors in `icons.ColorPalette`. It works fine, *except* if I change the background color of the project tree to a slightly lighter or darker color.

It seems to me that there's some sort of automatic contrast checker that disables my color definition if it fails to pass a certain threshold.

If this is true, is this documented somehwere on how it works? Or can I say in my theme to disable this?