If you are using theme_get_settings, such as: $value = trim(theme_get_setting('my_setting')); Then in your code editor you will also be seeing undefined  function 'theme_get_setting'.  This function has been deprecated for some time.   Instead, you need to use Passing the theme.manager and theme.initialization services to theme_handler constructor as optional parameters is deprecated. These parameters are also mandatory. $theme_handler = \Drupal::service('theme_handler');Get...

As a process I apply patches locally first, then using git upload the update(s) to a development site on the server.  Once the testing has been completed, the final step to apply the patch to the production site.   Using Terminal or your preferred shell program, navigate to the correct directory.  Then run this command using the name of the patch file (example.patch): patch < example.patchHowever, if you are patching Drupal core then remember to patch from the root directory...