Thanks to @chapel for sharing his customizations 
Is there a way to isolate the code that make the category bars coloured?
The css part would be something like this:
$color1: #30A92A;
$category-opacity: 0.3;
.category-bar {
margin: 0 !important;
padding: 0 !important;
width: 2px;
.category-meta & {
background-color: rgba($color1, $category-opacity);
}
}
…but i tried different combinations on the header part and i can’t get this to work…