Hi there,
Leo provides some CSS for changing the nav search colors here:
https://generatepress.com/forums/topic/navigation-search-colors/#post-1040744
To change the placeholder text requires this PHP snippet:
https://docs.generatepress.com/article/generate_navigation_search_output/#add-a-placeholder
For the related posts this would require a plugin, just search wordpress org plugins there are a lot to choose from that provide different query options.