Twenty Twelve: Header

Hi folks,

I’m customising the header of the Twenty Twelve template to match our website. I’ve made good progress with the header, however the rest of the WP style messes up (posts and side bar)

This is how it looks at the moment:
https://blog.lode.com.au/

I want to keep the post/side bar of the original Twenty Twelve style:
https://img16.imageshack.us/img16/6870/wordpressr.jpg

How can I achieve this original TwentyTwelve look while keeping my header intact?

Thanks for your help ??