To be totally honest, it's hard to convince to simply make the switch until one actually experiences it.
Dark themes don't trigger as much of the light senses in our eyes since we're viewing more dark areas over light(er) texts and elements, not to mention having spent lots of our times on the screen if it's part of your job. Done well and you'd be pleasantly surprised how good things look and become more immersive, with some degrees of battery saving on your mobile devices. Even helps with sleeping as well.
About the brightness control, that's just one thing but not it's not optimized on all devices. The opposite happens on one of my phones and I've to always adjust the slider to be brighter, only for it to revert mere seconds later.
Give it a try for one week (e.g. YouTube), I might see you on the dark side
EDIT: Forgot to add, reduces the burn-in screen effects