IMHO , Themes and Skins are a waste of time and fly in the face of css which
is an accepted and exceptionally usefull tool.

Master pages can be useful, but .NET does not allow you control over the ID
tag final naming if you are using them and controls, this makes life
complicated if you are trying to write css which conforms to accepted
standards where clustering of controls within a css class space is being
used..

For my money, unless you have to, stay away from skins make use of master
pages where it makes sense, and use css wherever possible.

.NET designer makes life hard on the CSS exponent because although you set
up a style sheet, as soon as you move something on the designer, you end up
with a bunch of element styles which overrride your css. So I have opted for
manual styling using css and leave the designer alone, this makes
development a longer cycle, but at least you have full control over the
layout through CSS and accessability is something which is easier to control
using style sheets with scalable measurements. Alternatively, what a lot of
designers do, is to forget all about the VS designer and retrocode the pages
with ASP.NET pagr directives manually etc. More troublesome, but at least
the control is with you not the VS designer.

if they really wanted to win a vote of confidence, then I would have
suggested that once a css is applied to a page, that repositioning the
elements was reflected int the style sheet declaration which last referenced
the element. That way you could have gotten the best of both worlds.

Thats my two cents anyway.