Header Edit Question

I’ve read through a number of replies on this subject but I couldn’t find an answer to my question.

Quite simply, I’m trying ot replace the default header in the default template with a custom header. I’ve managed to insert a custom header — but I can’t get rid of the large blue background that’s part of the default header.

Thanks for any help anyone can offer.

P.S. WordPress appears to be a mighty impressive tool.