Open the file style.css you’ll find in /wp-content/themes/autofocusplus/ and look for
/* Global
-------------------------------------------------------------- */
html {margin:0;padding:0}
body {color:#999999;background-color:#FFFFFF;font-size:12px;line-height:22px;font-family: helvetica, arial, sans-serif;}
[...]
in body {}
add background-image: url(‘https://www.WHERE_YOUR_IMAGE_IS/image.EXT’);
So you’ll have something like:
body {color:#999999;background-color:#FFFFFF;font-size:12px;line-height:22px;font-family: helvetica, arial, sans-serif;background-image: url('https://www.mysite.com/hello.jpg');}
For background-image properties (like position, repetition, etc…) have a look of: https://www.w3schools.com/css/css_background.asp