dim down
To lessen something in brightness, as of a light. A noun or pronoun can be used between "dim" and "down." I dimmed down the lights so that we could have a romantic candlelit dinner. We knew that the play was about to start once the lights dimmed down.
dim something down
to make lights dim; to use a dimmer to make the lights dimmer. Why don't you dim the lights down and put on some music? Let me dim down the lights and put on some music.
dim down
[for the lights] to go dim. The lights dimmed down for a few seconds. Open the stage curtain when the house lights dim down.