Add custom class

if you want to add custom class, you can do it by using F-builder. 

You can refer this article: 

https://www.magesolution.com/themes/claue/docs/m2/index.html#active-builder

https://arrowhitech.ticksy.com/article/12553/

Or you want to have custom css, please inspect it (by f12) find the class calling it out, then go admin -> theme setting -> choose the proper stroreview -> custom style -> add css -> clear cache -> check again in your private browser mode