If you choose any of the templates that have a banner you can also choose to upload an alternative banner which you can design yourself in a program like Photoshop.
The dimensions (in pixels) of the file you'll need to create are written next to the theme's title (see picture).
In the case of this theme it's 894 pixels wide by 129 pixels tall.
So in Photoshop you'd go to the "File" menu, select "New" and in the dialog box that opens up change the settings so they match the screenshot below:
Make sure that you save the file as a jpeg and also make sure that it has a filename with no spaces in it and the proper file extension at the end ".jpg" so for example save the file as: my_cool_banner.jpg
Then when you're selecting your theme you can click on the "banner image" menu (see image below) and select your new banner for uploading:
Don't forget to hit "Save" to make the changes permanent.