After you get acquainted with HTML, you should learn about CSS to learn to style your webpage. The goal of the HTML is to specify the content of your webpage while the goal of CSS is to style your web page.
CSS stands for Cascading Style Sheets. The full form gives an idea about it. A CSS file is a style sheet that specifies the style used by different elements. Some examples of properties of the elements that can be changed are margin, padding, color, font, and background-image.