Does everyone who’s following the old account automatically refollow you when you do that?
It doesn’t port over any old comments/posts, but I’m pretty sure that when anyone @'s you, it’s forwarded to the new account.
IMO it’d still be useful to be able to use an identity you control, like a domain name.
I would advise against using pixels for margin/padding since it’ll have issues for users who have different zoom/text sizes than you do.
Stick to rem for margin and padding.
If you’re still early days with css, it’s worth pointing out that you should use a “css reset” file. It will solve problems for you that you don’t even know exist yet.