Most companies that are implementing SharePoint request a customized look & feel in order to reflect the style and branding of their company. So design and branding is a key element in any SharePoint implementation.
A good understanding of the CSS styles used in SharePoint is mandatory for SharePoint branding. Now there is an excellent resource to dig into SharePoint 2010 CSS styles. Heather Solomon released SharePoint 2010 CSS Reference Chart that highlights the major styles used in SharePoint Foundation 2010 and SharePoint Server 2010.
CSS styles are grouped by area of the page and include the class/ID value, an explanation of how it is used in SharePoint and other related information including a screenshot where appropriate.
More resources about SharePoint 2010 Branding:
Heather Solomon’s SharePoint 2010 blog: http://blog.sharepointexperience.com/
Tom Wilson’s blog: http://styledpoint.com/blog/
Erik Swenson’s SharePoint branding blog: http://erikswenson.blogspot.com/
Nice branding samples: http://www.topsharepoint.com/