Hi Flavio,
X only has the default CSS codes that will correctly display up to 6 columns.
In continuation to what Prasant suggested, you will also need to add this custom CSS in X > Theme Options > CSS:
@media (min-width: 768px) {
.x-column.x-1-8 {
width: 9%;
background: center center/cover no-repeat;
}
}
Then to add a background to the columns, you will have to hard code the background setting from the HTML code you previously added. For example;
<div style="background-image: url(URL_OF_THE_BACKGROUND_IMAGE);" class="x-column x-sm x-1-8">Content here</div>
Kindly note that since this is a custom code that changes the default behavior/display of the theme, you will be responsible to maintain or update the code in case you require further changes or if the code stops working in future updates. If you are uncertain how to proceed, it would be best to get in touch with a developer.
Hope this helps.