The install_themes_{$tab} WordPress PHP action fires at the top of each of the tabs on the Install Themes page. The dynamic portion of the hook name, $tab, refers to the current theme installation tab.
Usage
add_action('install_themes_{$tab}', 'your_custom_function', 10, 1);
function your_custom_function($paged) {
  // your custom code here
}
Parameters
- $paged(int) – Number of the current page of results being viewed.
More information
See WordPress Developer Resources: install_themes_{$tab}
Examples
Add custom content to the Featured tab
Add custom content to the top of the Featured tab on the Install Themes page.
add_action('install_themes_featured', 'custom_featured_tab_content', 10, 1);
function custom_featured_tab_content($paged) {
  // Display custom content
  echo '**Welcome to the Featured Themes tab!**';
}
Display a message on the Dashboard tab
Display a message on the top of the Dashboard tab on the Install Themes page.
add_action('install_themes_dashboard', 'custom_dashboard_tab_message', 10, 1);
function custom_dashboard_tab_message($paged) {
  // Display a message
  echo '**Welcome to the Themes Dashboard!**';
}
Add a notice to the New tab
Add a custom notice to the top of the New tab on the Install Themes page.
add_action('install_themes_new', 'custom_new_tab_notice', 10, 1);
function custom_new_tab_notice($paged) {
  // Display a custom notice
  echo '**Check out the latest and greatest themes!**';
}
Display a banner on the Upload tab
Display a custom banner on the top of the Upload tab on the Install Themes page.
add_action('install_themes_upload', 'custom_upload_tab_banner', 10, 1);
function custom_upload_tab_banner($paged) {
  // Display a custom banner
  echo '**Upload your own awesome theme here!**';
}
Show a custom message on the Updated tab
Show a custom message on the top of the Updated tab on the Install Themes page.
add_action('install_themes_updated', 'custom_updated_tab_message', 10, 1);
function custom_updated_tab_message($paged) {
  // Display a custom message
  echo '**These themes have been recently updated.**';
}