The element class selector selects all the elements which match with the given class of the elements. Use the css() method to change the background color.
You can try to run the following code to learn how to select an element by its class name using jQuery:
<html> <head> <title>jQuery Selector</title> <script src = "https://ajax.googleapis.com/ajax/libs/jquery/3.2.1/jquery.min.js"></script> <script> $(document).ready(function() { $(".big").css("background-color", "yellow"); }); </script> </head> <body> <div class = "big" id="div1"> <p>This is first division of the DOM.</p> </div> <div class = "medium" id = "div2"> <p>This is second division of the DOM.</p> </div> <div class = "small" id = "div3"> <p>This is third division of the DOM</p> </div> </body> </html>