A bit late 
but maybe someone else needs it. I must mention that:
- Not all the CSS code is required, especially the borders and the background colors for table rows, so feel free to adjust them as you like.
- I had to change the layout from the regular table into flex cause you can’t have negative margins on table rows and other layout issues, and well…is more flexible
- Replace Category Group 1 and Category Group 2 with your desired category group names
- In my example I’ve used the category ID’s 2 and 4, so using chrome dev tools to inspect ( see pic below), find the right category id and replace
[data-category-id="2"]and[data-category-id="4"]in my example with your id(s).
Paste this inside the Common tab:
.category-list{
display: flex;
flex-direction: column;
}
.category-list thead tr{
display: flex;
}
.category-list .topics {
width: 95px;
margin-left: auto;
min-width: 95px;
white-space: nowrap;
}
.category-list tbody tr {
background-color: var(--primary-very-low);
box-sizing: border-box;
display: flex;
}
body .category-list [data-category-id="2"],body .category-list [data-category-id="4"] {
position: relative;
margin-top: 50px;
}
body [data-category-id="2"]::before {
content: "Category Group 1";
}
body [data-category-id="4"]::before {
content: "Category Group 2";
}
body [data-category-id="2"]::before,body [data-category-id="4"]::before {
position: absolute;
top: -35px;
left: 0;
font-weight: bold;
font-size: 15px;
text-transform: uppercase;
color: var(--primary-medium);
}
Paste this inside the Desktop tab:
.category-list tbody tr {
margin-bottom: 15px;
border: 1px solid var(--primary-low);
}
And this inside the Mobile tab:
body .category-list-item.category .posts {
width: auto;
margin-left: auto;
}
body .category-list-item>footer,body .subcategory-list-item>footer{
background-color: var(--primary-very-low);
}
.categories-list .category-list th {
padding: 0;
}
.category-list-item>footer .category-stat, .subcategory-list-item>footer .category-stat {
float: right;
}
.category-list-item>footer .category-stat:first-child, .subcategory-list-item>footer .category-stat:first-child{
margin-right: 0;
}
body .category-list-item.category tr:first-of-type{
padding: 10px 0;
border-bottom: 1px solid var(--primary-low);
}
body tr.category-topic-link:last-of-type{
border-bottom: 1px solid var(--primary-low);
}
body .categories-list .category-list th {
padding: 0;
}
body .category-list-item {
border-top: none;
margin-bottom: 10px;
}
body .category-list-item {
padding: 0;
border: 1px solid var(--primary-low)!important;
}
.category-list-item>footer, .subcategory-list-item>footer {
border-top: none;
}
And this is how it looks on desktop and mobile resolutions:
