[D8] Theme permission

Thank you for applying! I added the Git instructions for non-maintainer users and the PAReview checklist link. Reviewers will check the project and post comments to list what should be changed.

If you haven't done it, yet, please check the PAReview report and fix what needs to be fixed. There could be some false positives; verify that what reported is correct, before making any change.

Thanks for you contribution!

1. dynamicPermissions(): The machine name of permissions should be all lower case to be consistent with other Drupal permissions. "Edit Administration theme" has a couple of upper case characters in it.
2. RouteSubscriber: shouldn't you unset the old permission requirement on all those routes? Probably this works by accident now. Or do all the roles need the "administer themes" permission in addition?

Otherwise looks good to me, did not see any security issues.

Thank you for your contribution! I am going to update your account.

These are some recommended readings to help with excellent maintainership:

• Dries' post on Responsible maintainers
• Best practices for creating and maintaining projects
• Maintaining a drupal.org project with Git
• Commit messages - providing history and credit
• Release naming conventions.
• Helping maintainers in the issue queues

You can find more contributors chatting on the IRC #drupal-contribute channel. So, come hang out and stay involved.
Thank you, also, for your patience with the review process.
Anyone is welcome to participate in the review process. Please consider reviewing other projects that are pending review. I encourage you to learn more about that process and join the group of reviewers.

I thank all the dedicated reviewers as well.