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;
}