Well, Lemmy generally doesn't use much custom CSS outside of the standard Bootstrap 5 stuff, so writing one isn't too crazy.
You might find something here that floats your boat (you need the stylus extension): https://userstyles.world/search?q=lemmy (you might need to adjust on which pages these styles apply tho)
this is probably reddit's biggest PR nightmare and this will make a permanent stain on the company.
Lemmy/kbin getting increased development is probably the best case scenario.
with the reddit sync dev and other devs making 3rd party clients we're going to see Lemmy be a viable alternative in the coming weeks or months.
I've already stopped using reddit on mobile. if they kill old reddit then my usage drops by over 90 percent