How to change background color in visualforce page using slds class? -

i working visualforce pages. trying change background color of visualforce page. but, can't able that.

here's code:

   <div class="slds-color__background_gray-12 slds-p-around_medium">         <p>.slds-color__background_gray-12</p>                      </div> 

when trying above code not getting gray background "div" or "p" tag. appearing default white background itself.