Cascading Style Sheets, usually referred to as CSS are a mechanism for controlling the style (or rendering) of items onscreen, and indeed can be (broadly) applied to anything on screen.
The approach taken by CSS is independent of platform (Apple Mac, Microsoft Windows, Linux), browsers (Safari, Chrome Firefox) etc.
Numerous tutorials and references can be found for CSS on the wider web, via your favourite search engine.
In easy terms the CSS is a list of attributes, each separated by a semi-colon, each with a name and value (with a colon between).
CSS can be applied to a individual control on the screen, however, the better approach is to add to a separate file, and just the style is then named. This latter case is considered better as all the styling is in one place, removing duplication, and thus to change all the style of (say) all website buttons only one change is required.
Its called cascading as any style applied higher is automatically applied along with the specific alterations or additions, this can be within the CSS file itself, or a control within a site. Whole web sites and books have been written on the subject, so please refer to these for details of CSS itself.
In Rulevolution, styles can be added to an included CSS file for whole project (using the feature in the Website Node), with the name been applied to an individual component using the CSS element. Alternatively a CSS element may also specify the explicit CSS style.
A CSS element can be written by hand, or will be created automatically if the page builder (within Studio) is used.