The Lumieres used to be all unicorns, but the family has fallen from grace and now most of them are earth ponies, albeit taller and slimmer than their regular counteparts. Modern Lumieres specialize in hornless magic, and much like changelings, they must draw magic from a host. This host, however, doesn’t need to be another pony. It could be an artifact imbued in magic, but such thing has to be crafted specifically for that purpose, and has several usage limits. As such, they slowly leech magic from this external source and reutilize the magic within as if they were normal unicorns.
Centuries have passed since they saw the birth of the last unicorn in their family. Lumieres never really wanted to thin out their magic from their lines — they just took it for granted. Eventually, they woke up to find that the last of their unicorn in their family was near death. Legend says this last ancient of their line granted all of their descendants, (which he deemed careless) a way to compensate for their faults. In an act of compassion for his magicless grandchildren, he placed a spell to the younglings and sacrificed all of his magic and very own soul for it to happen. Thus, now all Lumieres can access magic and begin their lives with a tiny bit of mana for spending sparingly, which they should use during their childhood only to grab external magic — any kind of magic they are able to find is good. Since the first half of their childhood and as they grow, all Lumieres need a mentor or tutor from within their family because this mana spending is a disciplinary art rather than instinctual — (instinctual would be the case for unicorns possessing a horn)
The magic they produce reflects in their eyes, in any shade of yellow or white, and this earned their their name — because of their delatory luminic eyes, abnormal in pure earth ponies. All Lumieres have these kind of bright eyes. There are no pegasi Lumieres, but a resurgence of Unicorn Lumieres might be possible if one of them breeded with one. But due to the scorn of unicorn society towards these ponies, most unicorns are highly zealous of their own ranks, and this scenario is then highly unlikely to ever take place.
Most Lumieres learn to craft some sort of magical artifact in which they contain some ammount of spare mana just in case, and wear it with them in the form of jewelry. This is originally a very unicorn-exclusive tradition they have kept over centuries.
Even the most skilled Lumiere, even if his power rivalries that of some unicorns, must always keep their abilities to himself lest they risk the expulsion from their family and thus, total loss of magic. In such case, the expulsed member would be drained by all the other Lumieres of his magic to the last bit until is rendered totally powerless. He then could never regain his magic by any means, and will have to lead an earth pony’s life as it was originally intended for the dying lineage.