I've been playing around with some modifications to the default theme and saw something that seems off or maybe I didn't understand:

Acording to the documentation, custom themes must be placed in "styles/themes/mytheme/main.css". However, Zabbix searchs for the css at "styles" dir.

This is what I did:

At the file include/classes/core/Z.php I added the custom theme I made:

class Z extends ZBase {
public static function getThemes()

{ return array_merge(parent::getThemes(), array( 'custom-theme' => _('Custom Theme') )); }

}

Then I created the required directories and placed the .css in styles/themes/custom-theme/main.css.

But when I assign that theme to a user, I get this error:

"File does not exist: /usr/share/zabbix/styles/custom-theme.css".

So it seems that Zabbix looks for the css at the styles dir. This is different from what I understand from the documentation: https://www.zabbix.com/documentation/3.0/manual/web_interface/theming