How to make theme on mobile different on destop?

Answer 1

@media (max-width: 767px)
{
/* This works in smartphone only */

body { background-color:#000; }
}

/* 767, 480, 360 */

[ EDIT for @Apel ] -

Thank you for your completeness, i choose a CSS solution because it should be the easiest way to work with sizes ^_^

Answer 2

media queries with just width condition doesn't tell you that it is mobile device or not. it just tells the browser window size, which can be resized by the user.
what Maz answered is correct if you want to apply different style based on the browser window size