Enable Claro as an admin theme by default

[miromichalicka]

As an administrator, I want to use Claro for admin interfaces, so I can distiguish front pages and back office.

Approach:

• Add config file with theme settings.
• Add permissions to an "administrator" role

Acceptance criteria:

1. After logging in as a user with an "administrator" role I can use Claro theme for back office.