Gallery Test
Joomla! creates HTML pages that use Cascading Style Sheets to control the appearance of the page. This includes things like fonts, colors, margins, and background. The CSS files are part of your template.
When Joomla! creates a page, it creates different CSS classes that are then referenced in the CSS file to specify which style will apply to which parts of the HTML page. These class names are pre-programmed into Joomla!. But Joomla! allows you to modify or add CSS classes by way of the Class Suffix parameters. This lets you fine-tune the appearance of specific pages with no programming and very little work. The best way to understand this is to see specific examples.