Use media queries and make responsive design of your theme.

E.g., Aalborg theme is a responsive elgg theme.

Extend the /view/default/theme/css in your theme with the media queries. Look at this css as an example and a start point for you.

Hello Again. Any idea where are these defined in the default setup?

class="elgg-menu elgg-menu-site elgg-menu-site-default clearfix"

I only need to change them, but can't find their definition in navigation.css.php

Thank you

In first, look at this hook:

elgg_unregister_plugin_hook_handler('prepare', 'menu:site', '_elgg_site_menu_setup');

elgg_register_plugin_hook_handler('prepare', 'menu:site', 'my_site_menu_setup');

Learn more

In second, change CSS styles in:

views\default\elements\core.css.php

views\default\elements\navigation.css.php