In Drupal 8, I'm getting an exception 'InvalidArgumentException' with message 'The form argument system_theme_settings is not a valid form.'
The story is: I can't add custom colors to my installation profile because of the color module not setting the configuration until a form submit on the theme's settings is called, so I got the form_id with hook_form_alter(), added a custom form_submit handler and serialized the form_state variable into a file (because form_state was gigantic and I had no idea how to find the useful values in there) and loaded it with unserialize() in my installation profile. (Kind of reminiscent of this issue: https://www.drupal.org/node/1963922)
Then, I ran: \Drupal::formBuilder()->submitForm('system_theme_settings', $form_state_variable);
And was told by Drupal that 'system_theme_settings' isn't a valid form. But, how can this be true, if I already added a custom form submit handler to this form using this very form_id?
If I'm not supposed to use a form_id, but rather a specific function or something like that, where should I look to find this function?