Best mobile friendly theme for a blog

Can anyone suggest to me the best mobile-friendly theme, I am working with a client and their site is not mobile friendly, please have a look and let me know how to make it mobile friendly and which covers all the main elements of core web vitals.
Site: https://blackmartapk.co/

I see your site. I can’t find any issue in the apk @SophiaWannemaker.