I think your best bet here is to add your button classes onto the register links' skin ID in the theme.skin: <asp:HyperLink runat="server" SkinID="LoginControlRegisterLink" CssClass="btn btn-primary" />

Otherwise you can use CSS to turn the link into a button. In .LESS you can use .loginmodule .lnkregister:extend(.btn all, .btn-primary all) { // extra styles here } - but in pure CSS you'll just want to copy the button styles in under the right class name.

Hope this helps.