Решение:
Making ASP.NET Buttons Match the default WebSite Template.
<asp:Button ID="Button1" runat="server" Text="Button" CssClass="button" onmouseover="this.className='buttonhover'" onmouseout="this.className='button'" /> css: .button { color: White; background-color: #465C71; border:1px solid; border-color: Gray; } .buttonhover { color: White; background-color: #BFCBD6; border:1px solid; border-color: Gray; }