Adverbs usually end in -ly, so Brilliantly = adverb.
It has been a long time since I have read this book but I would say the moral of the Mirror of Erised is to not dwell on the past. To not focus on what you do not have and instead to focus on what you do have and make the most of it. Harry was so intrigued by seeing his family in the mirror that he no longer accepted that he was an orphan and Dum bledore had to put an end to it. Instead of dwelling on desires, make the most of what you already have.
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The theme is that ethnocentric hurdles are a problem Americans faced not only in the past, but still face today. Clifton and Dondre are not allowed to participate in major events with their peers due to the ethnocentric hurdles placed in front of them. For Clifton, this was not being allowed to go to an amusement park on his school trip. For Dondre, this was not being allowed to play in a golf tournament at a golf club that did not allow African Americans. Dondre and Clifton bonded over their shared experiences, and they and their classmates and teammates will be eternally bonded in their efforts to overcome the struggle of inequities that still rear up as the hideous face of hatred and discrimination.
The main theme that a reader can derive is that even though life can be hard, we need hope for a better tomorrow.