The problem with lxdm in Debian is that Debian forced a build against gtk3. The themes available are normally working for gtk2 only. My own distribution based on Debian sid also uses its own theme. I had to patch it to make it work in gtk3. What I did was looking at the code of Clearlooks, the default in Debian. Clearlooks was made ro work in both gtk2 and gtk3. Apply the changes they made in Clearlooks to your own theme and it will work again. Jeroen Diederen - MintPPC
