If there's any theme to this final part of the Dog Star Man saga or epic or Tone Poem to the Nature and Human Body itself, it's birth and rebirth. We see a baby being born in very quick fragments - it's clear it is a baby, the amniotic fluid can't be obscured no matter how many super-impositions there are - and then the man on the mountain needs to be reborn as well into... something more, I suppose(?) Whatever it is, this is a fine culmination of what Brakhage had on his mind at the time. According to other people online, at the time the filmmaker was in trouble as he was out of work and had kids before and another on the way. Sometimes if you're living in that part of the country of Colorado and the mountains it's more than likely to have that Sysyphus feeling of continually rolling the rock up the hill with little result except the continuation of that.
It may be hard to discern any 'theme' from this for some who are only in it for the visual pleasures and mind-f***ery. For the latter part that's certainly still here and in a fairly awesome way that's consistent through the other parts (and clearly if you made it this far you've at least been able to tolerate the other four parts, I include the prelude with that).
But I could see it as being about what it means to be a... human being, in essence: to create life, to witness life, to grow, to copulate, to climb a goddamn mountain and chop some wood. Oh, and to have Man's Best Friend at your side guiding you (or trying to lick your face until you get up again) certainly helps.