If you want the background color of the header or footer just as wide as the content of the email, you can change the header/footer width in Skins.
Go to Skins and the tab Style. There to Header and or Footer and change the width to 640px. Most boilerplates and our default boilerplate have this width, if you have another width at the content blocks, change the value to this width. Save and reload your boilerplate or email draft.
If you can't see any changes, you might have a project overwrite. Select the project you are using for your email draft in Skins and change or remove the value there.
You just want to change the width for a specific project, don't make the changes in Account settings instead choose the right project in Skins before you make any changes.
If you want to have the header/footer wider than the content of the email, or more specific, the background color of the header/footer, you can change the value in Width to 100%.