Theme Functions (functions.php) is the main file to add your custom code (in PHP language) to customize and/or change behavior of the theme or any plugin.
To add your code to functions.php go to Appearance > Editor then select Theme Functions file and paste your code in the end of editor and click Update File:
If there is a syntax error on that code, your site will stop working! Before making changes backup your functions.php file first.
If there is no Editor link under Appearance menu then you need to login to your FTP and navigate to ~/wp-content/themes/kalium/ (or /kalium-child) folder to add the code in functions.php file.
If you want to add custom code to the theme, then we suggest you to install and activate child theme. Otherwise, after each update of Kalium you have to re-insert that custom code you had previously used.