No estándar
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
Resumen
La pseudo-clase :-moz-lwtheme es verdadera y seleccionada en los documento chrome cuando el atributo lightweightthemes del elemento raíz es true (verdadero) y se ha seleccionado un tema