Basically Elgg has the same license as Drupal and WordPress, so this Drupal FAQ may help, and there are numerous resources about WordPress & Drupal licensing on the web.
Themes are like other types of Elgg plugins, i.e. you install them in the same way: http://learn.elgg.org/en/1.9/admin/plugins.html?highlight=themes#themes.
info@elgg.org
Security issues should be reported to security@elgg.org!
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