How to override Corev4.css?

   You can link your custom stylesheet to master page using CssRegistration "After" attribute will load your stylesheet after corev4.css as shown below.

<SharePoint:CssRegistration name="/Style Library/sitename/style.css"  After="corev4.css" runat="server"/>

You needn't copy the entire classes from corev4. Override only those whose styles you want to change, the rest will use default styles coming from corev4 or other default sharepoint css.

 

<SharePoint:CssRegistration name="/Style Library/sitename/style.css"  After="corev4.css" runat="server"/>