There are a lot of ways to this, but if you choose to do it with CSS, you'll have to trust the screen-width to identify the devices, and that can be pretty limited.
Even so, CSS solution is simple, and you can do it this way
CSS solution
According to this answer,
The current best way that I use to detect a mobile device is to know
its width and use the corresponding media query to catch it
Suggesting you do this:
/* Smartphones (portrait and landscape) ----------- */
@media only screen
and (min-device-width : 320px)
and (max-device-width : 480px) {
/* Styles */
}
/* Smartphones (landscape) ----------- */
@media only screen
and (min-width : 321px) {
/* Styles */
}
/* Smartphones (portrait) ----------- */
@media only screen
and (max-width : 320px) {
/* Styles */
}
/* iPads (portrait and landscape) ----------- */
@media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px) {
/* Styles */
}
/* iPads (landscape) ----------- */
@media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px)
and (orientation : landscape) {
/* Styles */
}
/* iPads (portrait) ----------- */
@media only screen
and (min-device-width : 768px)
and (max-device-width : 1024px)
and (orientation : portrait) {
/* Styles */
}
/* Desktops and laptops ----------- */
@media only screen
and (min-width : 1224px) {
/* Styles */
}
/* Large screens ----------- */
@media only screen
and (min-width : 1824px) {
/* Styles */
}
/* iPhone 4 ----------- */
@media
only screen and (-webkit-min-device-pixel-ratio : 1.5),
only screen and (min-device-pixel-ratio : 1.5) {
/* Styles */
}
css-source
Javascript solution
The other way to identify the device is using javascript. There is already a related answer telling how that can be accomplished.