Hooray! I just finished theming support of LXDM.
LXDM, the display manager of LXDE, now has full theming support based on gtkrc styles + GtkBuilder xml UI definition. No technology outside GTK+ iteself is needed and no additional library is required for this new feature. Now lxdm even comes with a default theme ported from gdm Industrial theme (See the screenshot).
The documentation for theming lxdm will be published later once things are settled. Now it’s themed with gtk pixmap engine with some dirty hacks, which is inefficient and quite limited. However, as I know, gtk-css-engine is being developed and it can be used to theme gtk+ applications with css syntax. So, later our gtkrc-based theming mechanism can directly benefit from gtk-css-engine and we can have more themes. So artists, stay tunned!