Change GTK theme for a single app

Question

I installed gelemental on my Xubuntu, which beautifully presents me the Periodic Table of Elements and gives information about each element.

However, on some themes (e.g Arc-s & Adwaita) the coloring is just non-existent, so I tried the methods here (for a local launcher) and here (for the launcher in /usr/...) in order to change it to Greybird theme.

It doesn't seem to work for me after all though. can anyone suggest a way to fix this?

Here is the local launcher I created (I tried spelling greybird with a lowercase 'g' as well, I made sure it looks how it want if I change the main theme itself to Greybird in the Appearence Setting):

[Desktop Entry]
Version=1.0
Type=Application
Name=Periodic Table
Comment=View the periodic table of elements
Exec=env GTK_THEME=Greybird gelemental
Icon=gelemental
Path=
Terminal=false
StartupNotify=false

And the launcher in /usr/share/applications/ (The original is now *.bakup):

[Desktop Entry]
Name=Periodic Table
...
Name[<Misc Languages>]=<Some Name>
...
Comment=View the periodic table of elements
...
Comment[<Misc Languages>]=<Some Description>
...
Exec=bash -c 'GTK2_RC_FILES=/usr/share/themes/Greybird/gtk-2.0/gtkrc gelemental'
Icon=gelemental
Terminal=false
Type=Application
Categories=GTK;Education;Science;Chemistry;

UPDATE: I found it probably has something to do with the XFCE special theming - but it's still weird, because the theming has to do with xfwm and the problem occurs when I switch it to Openbox as well (via openbox --replace). I still couldn't fix it in either of the two