Remove the first step (i.e.login step) from the checkout process in a particular them

I have two themes (mobile and desktop) for the single store view.
In mobile theme I want to remove the Login step which is the first step of the checkout process. But in desktop theme it will be as it is.
could anyone suggest how can it be done for specific theme ?

Thanks and Regards.

Onestepcheckout is a complicated task, you can use third party extension.