About CSS page layouts
From Joomla! Documentation
Professional websites separate styling from content. There are many reasons for this, the most obvious (to a developer) being the ability to control the appearance of many pages by changing one file.
Style information is coded in CSS, cascading style script, and usually stored in files with the suffix .css. A webpage contains a link to the associated .css file so a browser can find the appropriate style information. CSS can also be placed inside a html file between <script></script> tags
All CSS code is applied to an element of the html/xml page. If you want a style to encompass a whole page, you would probably apply it to the <body> element. You can apply styles to any html element such as,
