Is it possible to have different GTK+ theme in Unity and Gnome Shell?

Question

I sometimes use two different Desktop Environments like Unity and Gnome Shell (comparing, testing, creating screenshots, etc.) but Unity looks bad using Adwaita theme, which is the same with Gnome Shell and Ambiance theme. Changing every time is quite a hassle and sometimes (but now less frequent than before) it crashes (I'm using Oneiric).

Answer 1

You can try creating a script which you set to execute at each starts where you recognize which DE you are using and switch the theme in base of what you are running

Answer 2

Building upon the previous answers:

For 11.10

#!/bin/bash

if [ "$XDG_CURRENT_DESKTOP" = "Unity" ]
then
  gsettings set org.gnome.desktop.interface gtk-theme 'Ambiance'
  gconftool-2 --set --type string /apps/metacity/general/theme 'Ambiance'
  gsettings set org.gnome.desktop.interface icon-theme 'Faenza-Ambiance'

else
  gsettings set org.gnome.desktop.interface gtk-theme 'Adwaita'
  gconftool-2 --set --type string /apps/metacity/general/theme 'Adwaita'
  gsettings set org.gnome.desktop.interface icon-theme 'Faenza'

fi

For 12.04 and 12.10

#!/bin/bash

if [ "$XDG_CURRENT_DESKTOP" = "Unity" ]
then
  gsettings set org.gnome.desktop.interface gtk-theme 'Ambiance'
  gsettings set org.gnome.desktop.interface icon-theme 'ubuntu-mono-dark'
  gsettings set org.gnome.desktop.wm.preferences theme 'Ambiance'

else 
  gsettings set org.gnome.desktop.interface gtk-theme 'Adwaita'
  gsettings set org.gnome.desktop.interface icon-theme 'gnome'
  gsettings set org.gnome.desktop.wm.preferences theme 'Adwaita'

fi

Add this script to Startup Applications (Add => Command sh path/to/script).