I think you still should be able to do your own aspx/ascx modifications like you used to do. In fact with new Community server there is much more space for theme modifications.
As far as content fragments - I think you can even use your old aspx. There is new content fragment-related control class ExternalImplementedContentFragmentBase, so with it your own content fragment could be very simple to implement. For example:
public class MyControl : ExternallyImplementedContentFragmentBase
{
public MyControl() : base("~/Utility/ContentFragments/my_old_theme_file.ascx")
{
}
public override string FragmentDescription
{
get
{
return ResourceManager.GetString("MyControl_Description");
}
}
public override string FragmentName
{
get
{
return ResourceManager.GetString("MyControl_Name");
}
}
}
You can see lots of Telligent examples of those ascx content fragments at /web/Utility/ContentFragments. When you put your assebmbly into /bin system will pick it up. You even don't need SDK for this.
That's it, at this point your content fragment should be available for management at localhost/.../managewidgets.aspx and you can place it anywhere in /Web/Themes/generic/ just like it defined in default fiji or evolution2 themes.
Also you can play with new option to alter content fragments in theme.config.