How can I make this site look like the old SharePoint Overflow?
Until we're out of (or close to out of) beta, we will have this theme. The beta period began when we migrated from the old SharePoint Overflow to Stack Exchange. It will end with our site being launched if we continue to have good metrics - shown here - and when we complete various processes such as the 7 Essential Meta Questions of Every Beta - tag started here.
Obviously, theme work can be quite time and resource intensive, so we need to prove we are ready to launch before it can be done. (By the way, I have no doubt it will launch, but we have to follow the process.)
In terms of the current theme, it is supposed to look like a "draft" or "sketchpad" that's being worked on - because that's exactly what we're doing! We're building our community and what we want the site to be.