Found a good theme but not sure its mobile optimized

[supaul]

Unlike phpbb all the themes for latest version of mybb aren't responsive. Yeah, they may seem pretty good and also may have good loading speed in most of the devices but they are not fully optimized.

You have to contact the author of the theme. And ask him or her to make it responsive for you. Most of the authors will charge you a little money but its okay, in my opinion.