Syntax 

get_theme_mod(string $name, string|false $default = false)

Usage

get_theme_mod() is used to filter the theme modification, retrieve all the theme modifications.
Add this below code in wp-includes/theme.php to do the same.

<?php Function get_theme_mod($name, $default = false) {
	$mod = get_theme_mods();
	if( isset( $mod [ $name ] ) ) {
		Return apply_filters( "theme_mod_{$name}", $mod[ $name ]);
}
if ( is_string( $default ) ) {
        // Only run the replacement if an sprintf() string format pattern was found.
        if ( preg_match( '#(?<!%)%(?:\d+\$?)?s#', $default ) ) {
            $default = sprintf( $default, get_template_directory_uri(), get_stylesheet_directory_uri() );
        }
    }
return apply_filters( "theme_mod_{$name}", $default );
}

In header.php  add, 

<?php get_theme_mod( $name, $default );?>