Hi,
First create the section.
Edit that section and in the Style tab choose Background with the paintbrush and choose the color 'white".
This will be your white box.
Then you can drag the Title widget or the Text Editor widget (depending on your type of content) into the section and it should sit on top of the background. Of course, you will only know if the background is white if your page is set to a different color, or you add a colored border around the background for testing purposes.
I hope this works for you.