I had the same "you cannot create or install a new theme as your themes directory is not currently writable" problem.
But when I used an FTP program to change the Themes folder to writable, I got a new problem: As soon as the Themes folder is writable, my forum completely disappears. No matter what formum page I try to navigate to, I get a page that says "index". Nothing but a text file with the word "index".
This happens as soon as I change the Themes folder to writable. I don't even get a chance to install the new theme.
Any ideas what's causing this?
Thanks!