[GTK2] GTK2_RC_FILES override clashes with current theme

[lemonsqueeze <lemonsqueeze gmx com>]

Hi all,

I used to think it was enough to set GTK2_RC_FILES to override the
current theme for a given GTK2 app. And then I came across this:

Take ubuntu trusty, current theme set to Ambiance.
Say we want to run geeqie (gtk2) with a dark theme (AmbianceDark) from
  http://gnome-look.org/content/show.php?content=161649
Setting GTK2_RC_FILES doesn't work, Ambiance theme is still used.
(not a problem with the theme, works perfectly fine on its own)

It gets even weirder, trying to override it with other themes you get
into situations where it works (BlackMATE for example) or where both
themes get mixed up (?!)
So I started suspecting that both GTK2_RC_FILES and current theme were
getting used and somehow mixed up / clashing with each other. Indeed
strace shows both are getting read, and if access to Ambiance theme is
blocked it now works (!)

Best workaround I found so far is to set GTK_DATA_PREFIX to some dummy
value so that current theme's files are not found:
  GTK2_RC_FILES=/usr/share/themes/AmbianceDark/gtk-2.0/gtkrc
  GTK_DATA_PREFIX=""
Which seems quite a hack, not sure what other side effects this will have...

Question: Is this a bug or my understanding of how GTK2_RC_FILES works ?
Shouldn't GTK ignore the current theme altogether when GTK2_RC_FILES is
set ? Surely themes don't seem to be expected to use globally unique
style names ...

Cheers
--
lemonsqueeze