Generally, we use customized versions of themes and plugins to build applications without developing all features from scratch. Even in this book, we had to use a theme and some plugins while developing the major requirements from scratch. So, when using combinations of themes and other plugins, we tend to get styling issues as these plugins and themes are not fully compatible with one another. Therefore, it's almost a must that we customize features and designs of these plugins to create a uniform design.
Custom CSS plays a major role in customizing the styles of these themes and plugins. Until recently, we had to use a style sheet of child theme or external plugins to add custom CSS. WordPress 4.7 introduced a feature for custom CSS, where we can add CSS through theme customizer and preview the changes instantly. Let's look at how we can add and preview CSS using...