Making page all one color

How do I make my page all one color. I’ve changed everything to black on my Sydney theme page, except for text and still have a white border around my Gallery. Here’s a link to the page:
http://silverpennyproductions.com/PRINTS/ Thanks for any help.

Hi,

Please add these CSS code into: Customize > additional CSS.

.page-wrap .content-wrapper {
    background-color: black;
}

Regards,
Awan

Thank you, that worked great. I’m just surprised there isn’t a color button for that. Seems there is for just about everything else. Thanks again.