theme: Don't give login-dialog another background-color
Its parent lockDialogGroup has exactly the same background-color
set in the same file. Giving them both the same wasted render time on
overdraw, and caused multi-layer blending artifacts (slight flickering
of the grey background) when the login dialog fades in/out.
While the flicker can also be fixed using set_offscreen_redirect,
there's no point adding that overhead when the extra layer doesn't need
to be painted. Removing it halves the average render time of the login
animation.
Edited by Daniel van Vugt