What is photosynthesis anyway?
Hmm ... photosynthesis in and of itself is pretty specific: energy from sunlight is used to split water into oxygen, which is discarded as useless junk, and a bunch of hydrogen, with a bonus ATP (energy storage). The energy and hydrogen are then used to turn carbon dioxide into a carbohydrate, aka sugar. Hence photosynthesis ("light" + "putting-together"). Read up on it in detail; chemically it's amazing!
So what do you want to focus on?
Hey it's driven by sunlight!
There's some room to play with here. While not enough to completely support a human metabolism, it sure could slow starvation. Only those wealthy enough to have safe food supplies would go around wastefully wearing shirts.
Hey, it's all about the chloroplasts!
Supposedly chloroplasts were once (the ancestors of) cyanobacteria which got assimilated into a eukaryote. Who cares? We do! Because you can extend this to say that for your world we can do this with any prokaryote. Sky's the limit. There are:
Prokaryotes which metabolize sulfur. Now your humans can, too!
Prokaryotes which have toxic cell walls; now your humans are safe from predation!
Prokaryotes which produce vitamin C; no more scurvy for your mighty sailors!
And on and on and on.
Hint: almost all molecules in the entire biological world exist in prokaryotes (I read this a long time ago; I believe that the article said there were a few exotic snake venoms and compounds in flowers which were not found in prokaryota).
You can make these optional, nice-to-have features, or vital to life, such that sunlight-deprivation is a major crime and plot point.
They don't have to be bacteria that exist on Earth; make up your own! They don't even have to be light-driven. They could consume sugar (or other compounds) to provide some service... Maybe there's one which eats sugar and gives off oxygen, making for a race of awesome divers
Okay, now it gets real
So okay, we have humans which have assimilated some wacky bacterium to give them an advantage. Who's to say all humans assimilated the same one? There could be several bloodlines/tribes/clans each of which has a different symbiote. Which could mean:
Clan A will pay handsomely for a daughter of Clan B to marry into their line. Clan C may opt for a more sneaky approach.
Romeo can never marry Juliet because their symbiotes are incompatible; their children would not survive
"Skin color? You expect me to care about skin color? DEATH TO THE SULFUR METABOLIZERS!"