Just an addition to the previous comment, you can find layouts from the theme garden or from theme makers' blogs!

Also, for the layout of the dashboard, if you want to change it, you need an extension. I used to use it, but I don't know if extensions still work witg Tumblr.

Your blogs layout? o_o'
Go to your blog, in the right upper corner is a button called 'edit' - you can change layout there.

I don't think you can change your dashboard layout tho. If you can, I haven't figured out how.