Hi Koen,
This is not possible via the widget parameters, but you can use some custom css code like the following:
#mwall_items_WIDGET_ID .cat-CAT_NAME .mwall-item-inner {
background-color: red;
}
In the above code, replace WIDGET_ID
with the actual widget id, and CAT_NAME
with the category name.
For example:
#mwall_items_2 .cat-Action .mwall-item-inner {
background-color: red;
}