How to hook CSS file according to theme selection in the customizer section

You can access Customizer values with get_theme_mod(), so you just need to check the value of your setting before enqueueing stylesheets the normal way:

function wpse_310123_enqueue_scheme_css() {
    $scheme = get_theme_mod( 'theme_scheme' );

    if ( $scheme === 'green' ) {
        wp_enqueue_style( 'theme-green', get_theme_file_uri( 'green.css' ) );
    } elseif ( $scheme === 'blue') {
        wp_enqueue_style( 'theme-blue', get_theme_file_uri( 'blue.css' ) );
    }
}
add_action( 'wp_enqueue_scripts', 'wpse_310123_enqueue_scheme_css' );

Note that the above code assumes:

• The name of your Customizer setting is theme_scheme. If it’s not, change that to whatever your actual setting is.
• The possible values are green or blue. If they’re something else, like green-theme, then make sure to change those.
• Your blue.css and green.css files are in the root folder of your theme. If they’re not, add the directories to the file name, eg. get_theme_file_uri( 'assets/css/green.css' ).