So I've been able to have a good listen to the OST now and I really don't think there is an overriding "theme" there, certainly not as prominently as the previous OSTs.
the doodooodododooooooooooo doooooo bit from the
To Galaxy sample is in there a couple of times, I'd be happy if this became the new Halo theme.
Obviously the DUDUDUH DUDUDUH DUDUDUH is in there a few times, but I really don't think that's much of a theme.
I had a listen to the Halo:CE OST and it got me thinking that there are a number of recurring themes throughout Halo, all of which you could point to and say "that's the Halo theme". I suppose in time we may be able to do the same with Davidge's music. At least I hope so.
Also the Halo themes of old aren't in either the regular OST or the Remix album. There's hints to them, but the specific pieces don't feature at all, which is a shame imo.