Adding option for custom themes in Environment variables #435

Hello,

I've been trying to add custom themes as default.
By adding 'PMA_THEMEDEFAULT', to the $vars in config.inc.php and adding the code below to the bottom of this config file.

/* Configure custom themes as default */
if (isset($_ENV['PMA_THEMEDEFAULT'])){
    $cfg['ThemeDefault'] = $_ENV['PMA_THEMEDEFAULT'];
}

This made possible to create an ENV option like: - PMA_THEMEDEFAULT=blueberry to a docker compose file.
I think more people are looking for this option. Is it possible to add this to the main configuration?