The css and other skin files used are located at

/Data/Sites/[SiteID]/skins/[SkinName]

the css files are combined in the order that they are listed the style.config file in that folder.

After editing any css you should go to Administration > Advanced Tools > Design Tools > Cache Tool and click the button to reset the skin guid. This ensures the browser reloads the css instead of using its own cache.

Most of the skins we include were made in Artisteer. The latest versions of Artisteer target html 5 and therefore the IE 8 support may not be as good.

If you have a lot of IE 8 users to support you might have better luck trying some of the older skins included in the extra-skins.zip on our codeplex download page.