Additional features and minor customisations can be achieved via the use of custom code snippets.
Use the functions.php file
These snippet go in the functions.php file of your (child) theme. You can find more info on this in the WordPress codex here: https://codex.wordpress.org/Functions_File_Explained
Be careful though. Any syntax or coding error will make your site crash.
Use a functions plugin
Alternatively you can use a plugin instead: https://wordpress.org/plugins/my-custom-functions/ – which will not prevent the crashing.
Create your own custom plugin
Even better and more elegant. Construct this into a mini plugin.