The theme adds an extra field for a theme-specific option using the ACF or ACF PRO plugin.
Should the theme install the ACF or ACF PRO plugin itself?
Should a default for the option be used until ACF (PRO) was installed and user can select something?
The link you posted gives you the code you need to include ACF in your theme.
If you don’t want to include it twice, only require 'acf.php'
when: if(!function_exists('get_field')).
Should the theme - by best practice - suggest or even install ACF (PRO) on its own? Should it display a warning that ACF (PRO) isn’t installed - hence some features missing (the theme specific option fields)?
I don’t think it’s ever been suggested anywhere that a theme try and install a plugin. The most that a theme should ever do is say it’s suggested or required to install a plugin. It’s up to the user to actually do that though.