Question Custom Foldout Triangle color?

Is it possible to access the triangle that appears to the left of a foldout header in a custom inspector - you know the one the flips down when you open the foldout?

I seem to be able to style every aspect of a foldout except that triangle. Where in the foldout GUIStyle is it referenced?

The only place I see anything that looks like a triangle is in the scaledBackgrounds array in the normal, onNormal, and other groups. But replacing that with another Texture2D doesn't affect anything.

Thanks for you help!