ADE= Adobe Digital Editions
w3c=
http://www.w3schools.com/html/default.asp.
At the latter place you can learn about HTML and CSS. They have extensive tutorials about both. The stylesheet is a statement at the top of the HTML page or referred to there which governs how things should be displayed. The W3C pages are very good at explaining these.
How these statements interact with ADE and your reader, even when they are perfectly correct, may expand your mind to the cracking point. Good Luck!