Themes to be responsive

Hello,
So, we have a website that wasnt responsive..We dont want to change the theme for a responsive theme but we want to have it responsive..So, how can we include like a css on the header.php??And to be used it when show on different browser..As,we will be using media queries and we dont want to add it to the style.css but we want to create another css file and the style for resposive design will be called on there…

Also, like jquery or other js files..Cause we need like the navigation to be on a different style when shown on mobile…

Any suggestion on how??

Thank you..