Cascading Style Sheets (CSS) is the most widely used stylesheet language on the world wide web. It is standardised by the W3C and can be used to style any kind of XML document, including XHTML, SVG and XUL.
It was conceived to ease the separation of content (written in XML markup) and presentation (written in CSS) of documents. CSS can be used to define and alter almost all presentation aspects of a document, like colours, font sizes and types, text direction, element sizes and positioning, etc.
Joomla provides many class- and id-attributes in its XHTML output, which can be used to alter its presentation through the use of CSS definitions for said classes and ids.
CSS can also be used to improve accessibility of documents. It also provides mechanisms to present the same XML markup in different ways, optimised for different devices like screen-readers, speech-synthesizing browsers, printers, and Braille-based, tactile devices.
if you are interested in core joomla CSS, here are a few links to get you started: