Cascading Style Sheets are used to style the appearance of web page elements in a browser.
CSS properties allow you to define font settings, external and internal padding for elements, background images, and a huge number of other aspects of web page styling separately from the HTML code of the pages.
Joomla templates include CSS files containing the styling code for these templates.
Many elements of web pages generated by Joomla contain class and id attributes with defined values, for which you can apply your own CSS properties to customize the presentation of web pages.