I'd like to make the argument for ignoring themes altogether and using nothing but CSS. It's never been clear to me what value themes add; it's a very Microsoft-specific approach, and its output isn't always standards-compliant. By using CSS you will widen the pool of potential designers able to work on your project, and you will have a better chance of having a cross-browser and standards-compliant site.
If someone else is going to be styling this later, I'd just make sure that you provide enough "hooks" for them to be able to design this. This means adding CSS classes to pretty much everything you do that will be styled similarly, and wrapping things in divs or spans with CSS classes where appropriate, for example
<div class="ButtonContainer">
<asp:linkbutton runat="Server" cssclass="Button Cancel" command="Save" text="Save" />
<asp:linkbutton runat="Server" cssclass="Button Save" command="Cancel" text="Cancel" />
</div>
If you don't have a solid understanding of CSS and you don't have in-house naming conventions or standard style sheets, though, you won't really know how to structure your divs and classes. In our shop, the way we handle this is the programmer just writes standard ASP.NET markup, and the designer goes through and adds divs, spans, and class names that work with the style sheet they will develop.