I can't name one "theme" from TNG. Snippets, sure, but you're never going to have anything as iconic as the music from TOS.
"Name" or hum? I can't necessarily name every theme I know from TNG, but I can hum them. Off the top of my head, themes I can hum:
- The Borg theme from "The Best of Both Worlds"
- Picard's theme, taken from McCarthy's rejected theme music; I think that died out about season three though.
- Two themes from "Who Watches the Watchers".
- The theme from "Bobby Trap".
- Jones' Romulan theme.
- The flute theme from "The Inner Light", played in another episode and expanded with an orchestra on CD that was not used in any episode.
- Yar's funeral theme.
What I did hate, however, is when you had a cue that had orchestral and was nice or even grand, then it transitioned into synth, which is aggrieves to me.