It’s spesifically for Arthemia-Premium theme, when you see that there’s a category bar wich you can setting wich color for each category bar. There’s another style to make it look more better, that’s create image for each category bar. You can make it work by add this code below at functions.php

remove_filter('pre_term_description', 'wp_filter_kses');

After you add that line, you can see at category description for category setting, that you can input image url at this field.

The example site that use image for category bar.