How can I customise CSS for each category? (as in having different design for each category)

It’s a surprisingly simple customization — they’ve designed their base theme to work well with background images, and the images themselves are applied per-category (and the images are where most of the style comes from). This CSS accomplishes most of the work:


// This adds a background color to topic lists, so they're readable when there's a background image
tbody { 
    background-color: rgba(30,30,30,0.8);
}

// This adds a background color to topics that fades on the right 
.posts-wrapper { 
    background: linear-gradient(to right, rgba(31,34,37,0.95) 0%,rgba(0, 0, 0, 0.83) 80%,rgba(0,0,0,0) 100%);
}

body.category-staff { // Replace staff with the name of your category
   background-image: url(https://yourimagehere.jpg); // Add your category-specific background image
}