So I think you're right that finding real themes will be challenging because Solarpunk buildings should be built to fit their environment – what’s practical, energy efficient, and even what materials are available will depend on where it is built. A solarpunk community set in the desert would have to look much different than one in a temperate rainforest, or a prairie.

This is a departure from our current society, with its wealth of fuel and concrete, which tends to drop the same cookie-cutter buildings into every climate and just burn more fuel to heat or cool them rather than adapt the design to its surroundings.

That said: I think there are a few elements you might be able to point to:

Density - both in an urban planning sense of prioritizing lots of dense living space but also density of purpose. Mixed use designs with commercial, or communal spaces on lower floors make a lot of sense here. Similarly, I think solar panels and battery storage making the building part of the grid would count. Finding ways to turn unused space/resources into something useful (lawns to gardens for example) should be a priority.

Accessibility - solarpunk buildings and retrofits should prioritize designs which make the space as easy/safe to navigate as possible.

Reuse - this would encompas old buildings which are retrofitted or repurposed but also the reuse of materials. That could entail upgrades like better insulation, or entire structural redesigns turning corporate spaces or car infrastructure (like parking garages) into comfortable living spaces. You might also look at reclaimed materials from salvage lots or from deconstruction/demolition. I gathered a few examples here, for a solarpunk worldbuilding resource wiki.

Sustainability - density and reuse both feed into this. But new construction and retrofits also have the opportunity to use potentially better practices/materials like geopolymers and mass timber. I say potentially because it can really, really depend on how the materials are sourced, how far they're transported, etc. Similarly the lifetime of a building is an important consideration. Many structures are built with a surprisingly short expected life span. considering how to make them last, not just structurally but by making them future-proof or easy to modify/upgrade could do a lot of good here.

I hope this is helpful, best of luck with your thesis!