In general, it will require a change to the category.twig template file. Or in your theme's stylesheet.css file.
Override Engine * Integrated VQMod * Unused Images Manager * Instant Option Price Calculator * Number Option * Google Rich Snippets * Google Tag Manager * Export/Import Tool * SpamBot Buster * Survey Plus
If you can't find an extension to do the above, you could pay a developer such as ourselves or post a job in the Commercial Support Forum.
For quick, professional OpenCart support please email firstname.lastname@example.org