Hi, the following two snippets should do the trick
To get the default stylesheet that belongs to the Corporate Site:
SiteInfo site = SiteInfoProvider.GetSiteInfo("CorporateSite");
if (site != null)
{
    CssStylesheetInfo css = CssStylesheetInfoProvider.GetCssStylesheetInfo(site.SiteDefaultStylesheetID);
}To change the stylesheet for Corporate site:
CssStylesheetInfo newCss = CssStylesheetInfoProvider.GetCssStylesheetInfo("MyNewCSS");
if (newCss != null)
{
    SiteInfo site = SiteInfoProvider.GetSiteInfo("CorporateSite");
    if (site != null)
    {
        site.SiteDefaultStylesheetID = newCss.StylesheetID;
        SiteInfoProvider.SetSiteInfo(site);
    }
}