Both gtk2 and gtk3 are affected. oxygen-gtk with gtk2 doesn't have this problem.
This is a known issue resulting from upstream GTK removing theme engine functionality in GTK 3. There is some preliminary work on honoring color schemes dynamically using a script for both GTK 2 and 3 themes, but it is incomplete. Even when that work is done, integration and automation will require some cooperation which means it may only be available as an optional, manual feature when it first lands. However, it should be noted that this is very much against GTK's intended use so it's always going to be a bit of a hack underneath.
*** This bug has been marked as a duplicate of bug 356006 ***