Width leaderboard/header same as other regions/ rounded corners

You refer that may be minor issues will come up. Could you please elaborate on this?

Not without actually making the changes myself. The post that you reference give a fairly detailed overview of the steps necessary to make the changes that you seek... something like three pages worth. To give a more detailed description would essentially require that I just implement the customizations that you're looking to implement. And the specific details of such a change will depend largely on issues very specific to your customizations... things like the size of the background images that you want to use for the re-sized regions, etc.

If you attempt to make such changes and have specific questions about one detail or two, I can try to help. But if you need more direction than this, you probably need to just pay someone to make the changes for you. It's an hour or two of work for someone familiar with the nuts-and-bolts.