Instead of simply modifying the style.css file of the theme as other answers suggest, I would recommend taking full advantage of child themes. This way, it is possible to update the main theme (e.g. if security vulnerabilities are found or you just prefer to have the latest version) and also retain all of your modifications.
For example, if you want to modify the Twenty Fifteen theme, create a new directory /wp-content/themes/twentyfifteen-child/
* and in this directory you need a style.css
file with the following:
/*
Theme Name: Twenty Fifteen Child
Theme URI: http://example.com/twenty-fifteen-child/
Description: Twenty Fifteen Child Theme
Author: John Doe
Author URI: http://example.com
Template: twentyfifteen
Version: 1.0.0
License: GNU General Public License v2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
Tags: light, dark, two-columns, right-sidebar, responsive-layout, accessibility-ready
Text Domain: twenty-fifteen-child
*/
Modify accordingly. You can put whatever you want for anything, except the Template
line, which must be the same as the directory name of the parent theme. When using child themes, it will load any of the files in your new theme directory in addition to the ones in the parent theme. Specifically, styles.css
in the child theme is loaded after the one in the parent theme and functions.php
in the child theme is loaded before the functions.php in the parent theme. Any and all modifications to the theme would then be done to the files in the newly created twentyfifteen-child
directory.
* This directory can be called anything that you want, but this naming style is recommended since will make it obvious which theme is the parent.