The Deep Elves aren't a proper nation of peoples, as in the case of the Sand elves, Mountain Elves and such, but instead is a denomination that had been given to the descendants of surface elves that had been enslaved by peoples of the Underworld of Kazrrad -be Drows, Draak Harg or Dwarves-, and had never returned to the surface of their ancestors.
The culture and status of Deep elves depends much of the slaver society: normally under Draak Harg and Dwarven masters, their elven slaves remained segregated and were able to maintain many of their own cultural traditions, depending then on their ancestors.
In the case of Drows, being as well elves, the situation changed a bit: in most of the cases, they banned their elven slaves from practicing their religion and languages, being forced to speak drow languages and worship their gods -except in some cases-. In some drow cities, it was banned the union of drow and surface elves, but in others it wasn't -being normally frowned upon, though-.
In these cases of forced acculturation of elven slaves, often their Deep elven descendants would be culturally quite drow: as well, while it was uncommon the union of drows with captured and enslaved surface elves, it became more often in the case of deep elves to have relations with drow slaves and commoners of the lower castes.
While generally there was a prejudice amongst drow against the liberation of light elves, there would be emancipation of Deep Elves, who knowing only the reality of Kazrrad and the Drow cities, often remained in the drow cities: in some cases, these emancipated deep elves will further mix with drows, and in the case of the ones rich or respected enough will be called Light Drows, considered quite an achievement among Deep Elves.
However, normally the free Deep elves will remain in the lower end of the societal pyramid in Drow cities.
In general, Deep Elves had very light skin due the lack of sun, pale or dull hair and in the cases of Deep Elves of third or fourth generation -or more generations of deep elves- was quite common congenital blindness.