How to customize a popup

First of all we need to copy design and style files to our theme directory.

Copy from: app/design/frontend/base/default/template/mt/exitoffer/
Copy to: app/design/frontend/YOUR/THEME/template/mt/exitoffer/
Copy from: skin/frontend/base/default/css/mt/exitoffer.css
Copy to: skin/frontend/YOUR/THEME/css/mt/exitoffer.css

Change a background

If your goal is to change a background of popup, then open this file

skin/frontend/YOUR/THEME/css/mt/exitoffer.css

find the line:

 .mteo-theme-default {

after this line you need to add the background image css code:

background: url("http://www.wallpapersxl.com/wallpapers/1920x1080/art-fantasy/259997/art-fantasy-background-p-o-space-259997.jpg") !important;

Your code should look like the code below:

.mteo-theme-default {
    background: url("http://www.wallpapersxl.com/wallpapers/1920x1080/art-fantasy/259997/art-fantasy-background-p-o-space-259997.jpg") !important;
     width: 760px;
     height: auto;
     min-height: 480px;
     display: block;
     overflow: hidden;
}