You are not limited to the plugin’s defaults. The forms can be filtered with wpmem_login_form and wpmem_register_form, so you are free to use your own formatting or filter the existing formatting. Also, the functions are pluggable (although the registration form is a fairly large function to maintain if you decide to go that direction). See this post for more info on pluggable functions: http://butlerblog.com/2011/11/29/customizing-wp-members-with-pluggable-functions/
I was thinking more in the terms of a WP-Members plugin core change.
Most themes will be responsive in the near future due to the onslaught of smartphones and the younger generation choosing these devices vs a full desktop.
Suggest you work with the Responsive theme author Emil Uzelac to make the WP-Members plugin responsive.
Here is what Emil suggested when he assisted another plugin author with getting a photo management system converted to a responsive design:
“There’s no standard documentation to “Responsive Design” however it should be pretty straight forward change. All* fixed DIVs should be “converted” to fluid DIVs and that will turn fixed “layout” to fluid one.
Please see:
http://www.alistapart.com/articles/responsive-web-design/
and also feel free to peak at my http://ww.wp.xz.cn/extend/themes/responsive if needed.
Thanks,
Emil”