extend header template?

[BirdOPrey5]

As that is a custom style there is really no way to know what the style author used to control what you want. Best to ask from where you got the style, otherwise a link might help so we can bring up fire-bug and see what is controlling your margins.

I'm also not sure what you're asking- I see no repeating image on that screenshot- I think you want to extend that "bar" to the very ends of the page... but I'm not sure that is even the question.