Help with header customization

Nextcloud version:
Operating system and version: CentOS Linux release 7.4.1708
Apache or nginx version: Apache/2.4.6 (CentOS)
PHP version: 7.0.25

I am trying in need of inserting a banner image in the middle of the header. Been trying to fiddle with files in /var/www/html/nextcloud/core/css/ with no result.
Then I tried to create a custom theme based on “example” theme in /var/www/html/nextcloud/themes but when I added ‘theme’ => ‘mytheme’ in the config.php - the appearance didn’t change at all. My theme wasn’t applied. Not that it matters to me I 'm only looking for a way to insert that banner.

Thanks in advance.

Check this out:

Make sure you disabled the Theming app (on by default).

Start by testing the example theme. Just set ‘theme’ => ‘example’, then change to your own custom theme once you have that working.