A great series of articles explaining how to style SharePoint from http://www.cleverworkarounds.com which in itself looks like a useful place for all things SharePointy.
Remember to visit the other parts of the article.
Turns out that getting the CSS heirarchy right is a bit of a pain as you have little control over the position of CORE.CSS which will always gets rendered last so blows all your custom styling out of the water. So have soldiered on with the proposed solution in Part 3. Essentially using:
<SharePoint:CssRegistration name=”<%$SPUrl:~sitecollection/Style Library/zzzGlobalMaster.css%>“ runat=”server”/>
Where zzzGlobalMaster.css is my firm’s generic web styling. I’ve created a custom masterpage which makes us of the styles in this css. Note: This is using my own styles classes, I am not overwriting anything in CORE.CSS at this time. Then using the ‘master page settings’ inside SharePoint to specify another CSS stylesheet that handles specific styling for the particular site (which will include CORE.CSS overrides).