The widget come with no style at all, so that you can easily customize everything with your own css file.
Each DOM element of the widget has its own class, so everything is accessible in an easy way without the needs of complex selector.
As the widget is displayed in an iframe, your style is prevented of being in conflict with other stylesheet already loaded in the page.
In order to apply your own CSS style, add this following attribute to the widget script :
Here is an example of what you can obtain after some CSS customization :