This page explains how confluence changes the look and feel by modifying the CSS.
Cascading style sheets (cascading style Sheets (CSS)) are the industrialized standard for styling Web pages. The content of the Web page will be converted to HTML language, the display of the page, the appearance, etc. need to be defined through a CSS file. You can upload a CSS text file, or you can enter stylesheet directly after you apply the stylesheet to a space or the entire confluence site.
Description: By default, only system administration can edit CSS stylesheets for the entire confluence site. You want to allow any CSS style sheet with edit space with spatial administrator privileges to enter > Basic configuration > Security Configuration and then select the custom style sheet for the space (named stylesheets for Spaces).
Creating a CSS file and being able to apply this CSS file without bias to different browsers is a basic work of web design and also a fine work. It is a very important task to test the CSS modified confluence site to make sure that all the pages are displayed properly-for example, the modified CSS allows the confluence main panel to display normally, and also allows all basic pages to display properly.
To help you with this change, we created this help page: Basic styling Tutorial to give you a basic understanding of CSS.
Https://www.cwiki.us/display/CONFLUENCEWIKI/Styling+Confluence+with+CSS
Confluence 6 Introduction to using CSS style confluence