[Support]

Yes, you can edit theme background, fonts, etc..if you want. For example: to edit the background of your current theme, go to Style Administration > Themes > click the Edit button next to the name of your current theme (v3phpbb3). In the theme editor, find:

body {
/* Text-Sizing with ems: How to size text using ems | Clagnut § Typography · CSS*techniques */
font-family: Verdana;
color: #000000;
background-color: #E1E1E2;
font-size: 62.5%; /* This sets the default font size to be equivalent to 10px */
margin: 0;
}

Replace with:

body {
/* Text-Sizing with ems: How to size text using ems | Clagnut § Typography · CSS*techniques */
font-family: Verdana;
color: #000000;
background-image: url(http://abc.com/xyz.jpg);
background-color: #E1F245;
font-size: 62.5%; /* This sets the default font size to be equivalent to 10px */
margin: 0;
}

The download link you gave me does not works, can you please recheck it.