How to use particular CSS styles based on screen size / device

Question

Use @media queries. They serve this exact purpose. Here's an example how they work:

@media (max-width: 800px) {
  /* CSS that should be displayed if width is equal to or less than 800px goes here */
}

This would work only on devices whose width is equal to or less than 800px.

Read up more about media queries on the Mozilla Developer Network.

Detection is automatic. You must specify what css can be used for each screen resolution:

/* for all screens, use 14px font size */
body {  
    font-size: 14px;    
}
/* responsive, form small screens, use 13px font size */
@media (max-width: 479px) {
    body {
        font-size: 13px;
    }
}

@media queries serve this purpose. Here's an example:

@media only screen and (max-width: 991px) and (min-width: 769px){
 /* CSS that should be displayed if width is equal to or less than 991px and larger 
  than 768px goes here */
}

@media only screen and (max-width: 991px){
 /* CSS that should be displayed if width is equal to or less than 991px goes here */
}

I created a little javascript tool to style elements on screen size without using media queries or recompiling bootstrap css:

https://github.com/Heras/Responsive-Breakpoints

Just add class responsive-breakpoints to any element, and it will automagically add xs sm md lg xl classes to those elements.

Demo: https://codepen.io/HerasHackwork/pen/rGGNEK