How to include desktop css file only on desktop & mobile css file only on mobile?
Yes, you can do this with media queries in your info file.

Change...

stylesheets[all][] = css/style.css


to...

stylesheets[all and (min-width: 960px)][] = css/style.css 


That being said, you can repeat a lot of work doing it that way. It might be easier to declare all of your styles that you know will change in a responsive.css style sheet, and use

@media all and (min-width: 768px) { /* all of your styles here */ } 


At the bottom to change the values for a mobile version.