WordPress Custom CSS by Jetpack
Jetpack for WordPress launches a new type of feature which enables user to edit CSS themes without affecting your current themes CSS. It has the ability to combine your theme’s CSS and a custom CSS that you made. That means you just need to write what you want to change, you don’t need to copy all your theme’s stylesheet content.
CSS (Cascading Style Sheets) is a kind of code that tells the browser how to render a web page. You may delete these comments and get started with your customizations.
By default, your stylesheet will be loaded after the theme stylesheets, which means that your rules can take precedence and override the theme CSS rules.
The Custom CSS editor gives you the ability to add to or replace your theme’s CSS, all while supplying syntax coloring, auto-indentation, and immediate feedback on the validity of the CSS you’re writing.
To use the CSS editor, go to Appearance ? Edit CSS.