You can change the color or add an image of your choice to the header. To do this, open your Dashboard, click on Settings, then Widget, click on Advanced Settings, and then scroll down to the the "Design" section.
Changing the color of the header:
In the "Widget header color" field, put the the hexadecimal code of the color you want. You can find the hexadecimal code for any color here http://www.colorpicker.com.
Here is how it looks:
Adding an image to the header
You'll find below an example of the CSS code necessary to add an image to your header. Just adapt it with your image, and copy/paste it in the "CSS code to use in widget" text area.
background-image:url('INSERT YOUR URL HERE');
Put the URL of the image of your choice.
Here's how it looks:
And of course you can combine color and image!
If you know more about CSS, you can try to add other things! Feel free to customize your widget.