How to get random theme and save to variable

So I would like to get a random theme from config and save it on a variable then send that theme as a popup to player.

Iv tried putting

Code:

public $theme = [];

in the main.php file

then

Code:

$themes = array_rand($config->get("themes"));
$theme = $config->get("themes")[$themes];
$this->main->theme == $theme

then

Code:

$p->sendPopup($this->main->theme);

Anything I'm doing wrong or could correct as this does not work

 

PHP:

$this->main->theme = array_rand($config->get("themes"));