I see that bug in whatever version Lazarus I've got on Win 7 64-bit.
I looked at tabcontrol.inc in TTabControl.Paint, which looks something like this (comments added by me):
Details := ThemeServices.GetElementDetails(ttPane); // will set Details.Element = teTab
{ ... some rect code }
ThemeServices.DrawElement(lCanvas.Handle, Details, ARect2); // will paint the rect using clBackground (black on my comp)
So I think the bug is in DrawElement that should perhaps not be using clBackground but rather something else - like clBtnFace?
Possible change in themes.pas below. On my system it will fill the tab canvas with grey without themes and white with themes.
{...TThemeServices.DrawElement...}
teTab:
begin
if Details.Part in [TABP_PANE, TABP_BODY] then
FillWithColor(ARect, clBtnFace{clBackground}); //rr
end;
My concern is that the code in themes.pas seems quite old and this would have been noticed before because its so obvious (or is nobody using TTabControl and just using TPageControl).