Comment 5 for bug 172254

I don't know how to fix the problem, or even if I would call it a problem, but I believe the reason this is happening is that the Login Windows Preferences windows is being run by root and so follows the theme for root not necessarily your user theme. I assume that the root theme tries to mimic your theme and that's why it works for human and clearlooks (which are globally installed in /usr/share/themes) and not for T-ish (locally installed in ~/.themes). Try running other root-level programs (Synaptic, Users and Groups, Network, etc.) with the t-ish theme and compare with user-level programs.

One reason it might not be considered a problem is that using the root theme gives you a visual clue that the program you're using could has elevated privileges and *potentially* could be harmful if used incorrectly.

A possible workaround might be to install the theme (basically copy/paste) to /usr/share/themes.