This question appears at first glance to be a very simple one, but in reality it is not simple at all!

1. For the time being I am going to answer according to one approach adopted by the commentaries. This is based on the opinion, which may sound rather surprising, that the Mitzvah of lighting Shabbos candles is in fact a Torah Mitzvah and not a Rabbinical institution.

2. The Midrash Tanchuma at the beginning of Parshas Noach states that there are three transgressions, including that of the Shabbos candles, for which women die during childbirth (this is also mentioned, of course, in the well-known Mishnah in Shabbos 31b). The Tanchuma then states that all three of these are Torah transgressions. The Tanchuma writes that the source for lighting candles on Shabbos is the verse in Yeshayahu (58:13), "And you shall call Shabbos a delight."

3. One sees from Midrash Tanchuma that not only is the Mitzvah of lighting Shabbos candles a Torah Mitzvah, but also that the Mitzvah of Oneg Shabbos is mid'Oraisa. In fact, if we look at the very first Mishneh Berurah in Hilchos Shabbos (OC 242:1), we find that some authorities maintain that the Ikar of the Mitzvah of Oneg Shabbos is from Torah law. The Sha'ar ha'Tziyun (#1) cites the Ramban in Parshas Emor as the source that Mitzvas Oneg Shabbos is mid'Oraisa. The Midrash Tanchuna is a good proof from Chazal for the Ramban (who is a Rishon) that Oneg Shabbos is a Torah Mitzvah.

4. However, even according to the opinion that the Mitvah of lighting the Shabbos candles is a Torah Mitzvah, the Berachah made on it is mid'Rabanan. The source for this is the Gemara in Berachos (33a) that says that the Anshei Keneses ha'Gedolah instituted the Berachos for Klal Yisrael.

5. I found another Chidush in one of the Rishonim, the Sefer ha'Yere'im, by Rabeinu Eliezer from Mitz (chapter 412(99)). The Sefer ha'Yere'im counts the Mitzvah of Oneg Shabbos as one of the 613 Mitzvos of the Torah. He writes that this Mitzvah was "Gemara Gemira Lah" (i.e. it is a Halachah l'Moshe mi'Sinai) until the prophet Yeshayahu came and gave support for this in a verse ("And you shall call the Shabat a delight" -- Yeshayahu 58:13).

6. Since the Rambam (Hilchos Shabbos 5:1) writes that lighting Shabbos candles is included in the Mitzvah of Oneg Shabbos, it follows that according to the Sefer ha'Yere'im, lighting candles is a Mitzvah given to Moshe Rabeinu at Har Sinai.

7. This is stated explicitly in Chidushei Chasam Sofer (Shabbos 25b, end of DH v'Lulei), who writes that he found in Sefer ha'Yere'im (ch. 99, 100) that both Oneg Shabbos and lighting candles are Mitzvos given to Moshe Rabeinu at Sinai. Later on the prophets found support for this Mitzvah in the verses.

Kol Tuv,

Dovid Bloom