Custom Smilies WP Theme

Hello, I am making a WordPress theme and I have custom smilie emoticons that I want to be used whenever someone uses the theme. I know I can just replace the default ones, but I want them to be the ones chosen when my theme is activated. How can I do this? I was thinking the functions.php file, but I am not sure. I am new to PHP and WP, so please explain. Thank you!