\#BW LESS-CSS is an easy to use interface for using LESS-CSS stylesheets in your theme.
The 'mobile' checkbox marks a file as mobile, meaning it will only be included when your website is viewed on the devices checked under 'Detect mobile for.' If you also specify custom media for a mobile stylesheet, that file will be included on non-mobile devices, for the purpose of responsive design. You can specify which devices you want to serve mobile to under 'Detect mobile for'
Yes. To make use of the mobile detection, including device-specific detection under 'Detect mobile for', call bw_is_mobile(), which will return a boolean (true or false).