Their markup is different partiall because they haven’t appeared to update their site in quite a while
Adding this CSS gets you most of the way there with the subcategory style…
.category-list .subcategories .subcategory {
display: flex;
}
If you want to make markup changes, you’ll have to start looking into how template overrides in Discourse work… that’s in the advanced section of Developer’s guide to Discourse Themes