LoginRequired plugin is available up to Elgg 2.0. You just have to check the plugin page for the newer versions. It should also not disturbe the look provided by a theme in any kind. Another matter is the content within the landing page though. If you have a specific landing page with custom content, it's likely you would have to make some modifications either within the LoginRequired plugin or within the plugin providing this landing page to get the desired output instead of the index page provided by the LoginRequired plugin (which is basically just overriding the index page provided by the default custom_index plugin).