Late Reply to this, and this is really good, and informational but I believe some minor edits for future uses will be needed to correctly function (as php and html need to be separated slightly) how does this work for ya?
function plan_description(){
/* create html block for before main container*/
echo '<p>Hello World</p>';
Thanks for the information and take care!!
well the quick answer is the <body>, the longer answer.. is that you might need to delve into how your theme set it up. It will all come back to the body regardless, but it is best to target the child if there is any specifically to ensure the results you want are met.
example body {background: url("image_url_here.jpg");}
To add, I would recommend you tell your browser what padding size your using if not a %, px, em etc
