Something like this:

elgg_register_plugin_hook_handler('vars:compiler', 'css', 'my_theme');

function my_theme(\Elgg\Hook $hook) {
   $vars = $hook->getValue();

   $keys = [
      'body-background-color' => '#ffffff',
      'font-size' => '16px', 
   ];

   $vars = array_merge($vars, $keys);

   return $vars;

};

Thank you RvR!!

I have solved it

Thanks Rohit!

I activated only Tidypics plugin when testing my Elgg3.2 site but the plupload-not-defined issue still exists.

I will look into it once I get free time.. probably in this weekend.

Thanks Rohit!

Are group notifications (email method) enabled for the group and the member account you are testing the sending? There're setting pages for notifications and group notifications each user can configure on his own (Settings menu in topbar and then the two notification menu entries in the sidebar will bring you to these pages).