We find the concept of hidur by rabbinic mitzvos. For example menorah. I saw some paper qoute the rema that one should get new aravos for the 7 days of sukkos because of hiddur. The 7 days are also rabbinic. My question is, if you understand the obligation of mitzvos diraban is because of lo tasur (don't stray from what they tell you) meaning it's a lo taseh, if so what does one gain by doing hidur? Even if you don't do hidur you still didn't go against what the rabbis said.
The Netziv writes based on the Rambam that there is also a positive obligation to obey the Rabbis, from the beginning of the verse אשר יאמרו לך תעשה (that which [the Rabbis] say to you, do.) Otherwise, one would not be able to make a blessing on a Rabbinic mitzvah, as blessings are only said on positive commandments. Therefore, one can do a hiddur in fulfilling the positive commandment to listen to the Rabbis.
A second approach can be based on a principle of Rav Shlomo Zalman Auerbach (published in the sefer Tehillah l'Yonah to Maseches Megillah) that the commandment of Lo Sasur gives the Rabbis the authority to create commandments, even on people who are not obligated in other Mitzvos (like minors). So once the Rabbis create the mitzvah of lulav on all seven days, it could now also have hiddurim.