It looks like you’ve managed to change the borders above and below the header.
As for the date, that is a sprite image.
Three options for changing it:
1) Download the .psd file, edit it in Photoshop, save it as a png and upload it to your blog.
2) Create a custom background image, upload it, and adjust your css to use that image instead of the default sprite image for the background of the date.
3) Adjust your css so that it doesn’t use a background image at all. (I have seen some people do this, and it offers a lot of creative options.)