You can easily update the changes in css or any of the application in browser for testing, once you satisfied the result then you can implement the same in your application.
If you want to know how to use, then press F12 in Keyboard and do changes as you want.
Refer below attachments, here you can easily identify the difference I change the Font size to 35px;-------------------------------------------------------------------------------- Give respect to your work, Instead of trying to impress your boss.
It can be done in the following way: You need to use Control - Shift - J and Developer Tools will open. Now you need to focus on the Console. This is for Google Chrome.Thanks