Comment 92 for bug 527267

Found good workagound - crate ~/.gtkrc-2.0 file with content:

----------- cut here -----------
style "mythin"
{
 xthickness = 0
 GtkMenuItem::horizontal-padding = 2
}

widget "*fast-user-switch*" style "mythin"
----------- cut here -----------

You may play with numbers to achieve acceptable result.