How to add external css in visual webpart in SharePoint 2010

If you want to add external css on visual webpart,

1. First go to layouts folder in 14 hive, path is C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS
2. Create a new folder in it and give a proper name to that folder.
3. Add your css files in that newly created folder.
4. Now to call the css file from your webpart code, add below tag in your ascx file <SharePoint:CssRegistration ID="cssReg" runat="server" Name="/_layouts/ForCss/style.css"></SharePoint:CssRegistration>

Done. Hope this helps.