If you are using a responsive template, you can use the Style Designer to change the background of the banner area. Please do the following:
- Go to Style > Designer.
- Under Colors, click Banner Image.
- On the dialog box that opens up, click Background Color.
- You can enter the hex color code or use the Color selector tool to add in the new color.
- Click Save.
If you are using a non-responsive style and place a custom banner in the banner area that is not the full size of the banner area, white or black color will fill the rest of the area. This is how the banner area is set to function. The best thing to do is to create a banner that is the exact size of the banner area. You can find the exact dimensions by mousing over the banner area. At the top of the banner area, you will see the banner dimensions.
There are quite a few banners making programs on the Internet if you would like to create another one:
Here is a link to our public forum where a group of users were discussing this topic and gave lots of good ideas: What is the best banner making program?
You may also like:
Create various banner sizes with the Custom Panel Widget http://www.yola.com/blog/5-minute-website-fixes-create-various-banner-sizes-with-the-custom-panel-widget/