How to override a function call in functions.php?

In functions.php of a theme makes a call to show_admin_bar(false) which hides admin bar in front end:

if (!is_admin())
{
    wp_deregister_style( 'bp-admin-bar' );
    if ( function_exists( 'show_admin_bar' ) )
    {
        show_admin_bar( false );
        remove_action( 'bp_init', 'bp_core_load_buddybar_css' );
    }
}

I want to make admin bar appear in front end for the admin users. To do this, I added the following code to a plugin:

add_action('plugins_loaded', 'show_admin_bar_to_admins', 100);
function show_admin_bar_to_admins()
{
    if (current_user_can('manage_options')) {
        show_admin_bar(true);
    }
}

But this call didn’t make a difference. I put 100 as the priority value such in order to make this function to be called later than the call inside functions.php. But it didn’t make a difference.

Is there a way to make a function call inside a plugin to be executed later than the call inside functions.php.

First off: This Theme is so doing it wrong. One should not simply stuff plain calls in functions.php files. Those should be wrapped and hook. Best to after_setup_theme(). You could btw try the same hook.