You can use the CSS section in the theme editor to add custom CSS
Alternative solution:
PS The template with subcategories is located here
design/themes/responsive/templates/views/categories/components/subcategories.tpl
You can use the CSS section in the theme editor to add custom CSS
Alternative solution:
PS The template with subcategories is located here
design/themes/responsive/templates/views/categories/components/subcategories.tpl