Currently background tabs are painted using COLOR_BACKGROUND_TAB. Instead they should be painted using BrowserNonClientFrameView::GetFrameColor().
One way to implement this might be to change MaybeGetDefaultColorForNewerMaterialUi() in theme_properties.cc to call this. But that would have to be able to get at the current frame, which would be challenging.
This bug does not cover making the text in these tabs readable, just getting the right background color.
Comment 1 by pkasting@chromium.org
, May 31 2018