If you want your widgets by Personalized Recommendations to match the theme of your store by changing the background colour of "Add to Cart Button" , you can do this easily using the Visual Editor.
Go to your Personal Recommendations Dashboard and find the widget you want to customize.
Now open the Visual Editor to get live previews while you edit.
Here is a video demonstrating how you can use it:
You can use this Link to find appropriate colour codes to fill in.