Cascading Style Sheets is the way to go. Change GUI look by just changing one file, the CSS file.
David Sawyer McFarland, August 24, 2009. Second Edition, 560 pages, Pogue Press, 0596802447, 978-0596802448, The O'Reilly Missing Manuals Series.
A lot of revisions of the previous edition. Part one, the strong case for using CSS instead of the old <font> </font> every few sentences. The internal style sheets for each page vs an external style sheet for each look of your whole web site. Stop a lot of typing. Make it easier for you to understand your own web site.
Part Two, formatting text, margins, adding graphics, navigation.
Part Three, how to layout your pages.
Part Four, style sheet for printing.