That just looks like the text box has a border-radius (CSS3) attribute as well as some background colouring.
I'm unsure about the template, possibly the header template if the login displays on every page.
vBulletin Options -> General Settings -> Add Template Names In HTML Comments
Now view the source, find the login box, then scroll up and look for the first piece of code that looks like this:
HTML Code:
<!-- BEGIN TEMPLATE: templatename -->
Now go to that template and edit to your heart's content.