Make sure your style.config has this in it like all the other skins do:
<file csswebconfigkey="YUITabCss" imagebasewebconfigkey="YUISkinImagePath">none</file>
<file cssvpath="/ClientScript/oomph/oomph.css" imagebasevpath="/ClientScript/oomph/">none</file>
and make sure you have this in Web.config:
<add key="YUIBasePath" value="~/ClientScript/yui-252/" />
<add key="YUITabCss" value="~/ClientScript/yui-252/assets/skins/sam/tabview.css" />
<add key="YUISkinImagePath" value="/ClientScript/yui-252/assets/skins/sam/" />
You can debug the css by manipulting these settings:
<add key="CombineCSS" value="true" />
<add key="MinifyCSS" value="true" />
you have to both clear the server cache by touching Web.config and the browser cache for changes to these settings to take effect.
you can for example set MinifyCss to false, then view the source of the page and grab the full url for csshandler.ashx and paste it into a new browser tab to see the css un minified.
After resolving any problems I recommend set both settings back to true.
Hope it helps,
Joe