Hi Massi,
You would have to add it. What we're going to do for the next release is add SkinIDs to the links on the login page. You can go ahead and do this yourself and then use the theme.skin to add the bootstrap button classes.
In /Controls/LoginControl.ascx, go to the two <asp:HyperLink controls towards the bottom of the file and replace them with this:
<asp:HyperLink ID="lnkPasswordRecovery" runat="server" CssClass="lnkpasswordrecovery" SkinID="LoginControlPasswordRecoveryLink"/>
<asp:HyperLink ID="lnkRegisterExtraLink" runat="server" CssClass="lnkregister" SkinID="LoginControlRegisterLink"/>
Next, in your skin's theme.skin file, add the following.
<asp:HyperLink runat="server" SkinID="LoginControlPasswordRecoveryLink" CssClass="lnkpasswordrecovery"/>
<asp:HyperLink runat="server" SkinID="LoginControlRegisterLink" CssClass="lnkregister btn btn-warning"/>
You can change the btn-warning to btn-default, btn-primary, btn-danger, btn-info, or btn-success.
Hope this helps,
Joe