You may upload your own header/footer graphics. In order to replace the existing one all you need to do is upload the desired image to the image manager in your pages section with the same name as the original.

The simplest way to do this is to edit the existing graphic. You can download the image by right clicking it and saving to your computer. Then edit that image to look however you’d like it to. This will keep the image dimensions the same as the theme default. Use the Image Manager under the Pages tab to upload your edited image into your images folder. Your uploaded version will overwrite the default image.