I admire the way your phrased your final questions. It is clear that you are open to thinking about things differently than you have in the past. And that's exactly what you have to do when you start using the CSS Friendly adapters.
Rather than thinking about styling by using classes that you set with the traditional CssClass attribute, you want to start thinking about using more "tag based" CSS selectors. This is an enormous conceptual jump for a lot of web authors... but once you make it, you never go back! Light some candles, put on some relaxing music and try to get into a good mental space. Now clear your mind and imagine a blank piece of white paper where we are going to sketch out some things. First, imagine that you see you <asp:Menu>. Now imagine that it transformed into the HTML that the adapters will produce for that menu. That HTML is laid out in a really simple and predictable way. On the outside is a <div> whose class equals the value you set for the CssSelectorClass of the <asp:Menu>. Inside that <div> is a <ul> whose class is AspNet-Menu. It is the "main" unordered list (ul tag) for your <asp:Menu>. Inside it are, naturally, some <li> tags that are the menu items of your menu's first tier. If your menu has submenus than some of these <li> first tier menu items will contain <ul> tags (that themselves contain more <li> tags, etc.).
<div class="someCssSelectorClass">
<ul class="AspNet-Menu">
<li>...</li>
<li>
<ul>
<li>...</li>
<li>...</li>
</ul>
</li>
<li>...</li>
</ul>
</div>
So you can now build CSS rules whose selectors "target" any part of this HTML reliably. Suppose you want your "static" (1st tier) menu items to have a blue background and all submenu items (in the 2nd, 3rd, etc. tiers) to have a green background. Then you can use a doublet of CSS rules like this:
.someCssSelectorClass ul.AspNet-Menu li { background: blue; }
.someCssSelectorClass ul.AspNet-Menu li li {background: green; }
The first rule says that the background of all list items (li tags) that are children, grandchildren, greatgrandchildren, etc. of the topmost ul in the menu should have a blue background. The second rule overrides that color with green for the grandchildren, greatgrandchildren, etc. of the topmost ul. That's a common pattern of implementation in CSS. Rules are often in doublets or triplets, etc., so that the earlier ones are refined by the later ones. The later rules tend to have selectors that impact let of the DOM in the HTML so that your rules stack up (cascade) much like painting a pictures by adding layers. You pain the whole canvas the color the sky and then add mountains in the background on thop of the sky. Then you add trees and snow on the mountains, etc. Each layer is smaller than the one before it. CSS selectors are stacked in an identical fashion.
The important thing here is that you get the idea that the adapters give you a very simple and predictable set of rendered HTML tags for a menu or treeview, etc. That predictability lets you stop using CssClass as much and start, instead, writing CSS rules (selectors) based on the foreknowledge of the rendered HTML. That is a radical departure from the traditional ASP.NET philosophy that you should never count on the predictability of the rendered HTML. Here, we are telling you that that is OK to do in order to let you create style sheets that are more "tag based" in their CSS selectors. By doing so, you no longer need to scatter CSS classes over your markup in your ASPX files. You basically end specifying the outer <div>'s class via the CssSelectorClass you use on your tag (like your asp:menu). From that and the simple foreknowledge of what the adapters produce you can do everything you want in CSS, cleanly decoupling your markup and your styles in a skin-able CSS fashion.
Once you get that whole pattern, it makes a lot more sense when you go and study these pictures (that my wife, Heidi, brilliantly composed) http://www.asp.net/cssadapters/whitepaper.aspx#SamplesCSSClassesMenu.
This is a pretty huge jump in thinking so if you are still fuzzy on stuff, write back and I'll try to coach you along more. Good luck and Happy New Year.
Russ Helfand
Groovybits.com