Hi, the existing text in the CSS window is just informational and the code would have to go below that informational text, or the informational text would first have to be deleted. I’ve done that for you and am seeing the white background now on your widgets and posts. Take a look and see what you think and let us know if you wish to make any other adjustments.
Here is a site with a websmart color wheel. You hover your mouse and when you see the color you want, click your mouse and it will enter that hex color number in the panel to the left and then you can copy it out and use it.
This looks like it might work well with your site. Replace the #FFFFFF in the background declaration with #EEEEFF and see what you think.