Code to run on WP theme activation and deactivation.

switch-theme.php

<?php

/**
 * On theme activation.
 */
function ivanhoe_after_switch_theme($theme_name) {
    
}

add_action('after_switch_theme', 'ivanhoe_after_switch_theme');

/**
 * On theme deactivation.
 */
function ivanhoe_switch_theme($theme_name) {
    
}

add_action('switch_theme', 'ivanhoe_switch_theme');