Non-standard
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.
Summary
The :-moz-lwtheme-darktext pseudo-class matches in chrome documents when :-moz-lwtheme is true and a lightweight theme with a dark text color is selected.