In this tutorial, we'll cover how to do five quick CSS customizations that make your blog stand out. Even if you don't know what CSS is, you'll be able to follow the instructions and change the standard look of your theme. Before we start, you need a theme with custom CSS options or a custom CSS plugin, or a child theme.
If you don't already know how, Ragnar Miljeteig will show you how to find and set up these things. That way you can safely edit the layout of your website or blog. If your website is already live and getting traffic, it can be a good idea to use a "down for maintenance" plugin so visitors don't become confused by the ever-changing layout.
Another alternative would be to set up a development environment on your local machine, and then upload the changes to the server once they are completed.