As done by others, implement the option that a default forum theme can be set by the Admin for mobile visitors.
Why that, elkarte is already responsive, so you don't need different theme for mobile
True that ElkArte has a responsive theme but it will not be the only theme that is used by different sites. A site Admin or owner may choose to have one custom theme for a desktop and a different custom theme for mobile.
As well, a site Admin could have perhaps 5 custom themes for mobile and they want to permit their users to select a theme that the user prefers.
So, it can be an option for only the Admin to set for all mobile users or it could also be an option that the Admin permits 'x' number of available themes for mobile members/guests to use. It is really about offering choice and in the mobile world that kind of choice is a plus for ElkArte.