Drupal Gardens: is it possible to install a new theme? I need to create a site into Drupal Gardens, but my client needs a custom responsive theme. I have searched in the FAQ and internal page of Drupal Gardens, but I have not found anything about this possibility. 

Is there a way to install a theme for my site into Drupal Gardens?
Can I install some modules from drupal.org?
Drupal Gardens is really limited and you cannot install a theme from drupal.org but what you can do is to customize the possible "layouts" that Gardens give to you.

If you click on the layout button on the panel you will see different layouts (columns and sidebars) and from this basic structure you can start applying custom CSS. Check more here http://www.drupalgardens.com/documentation/themebuilder/layout-tab and http://www.drupalgardens.com/documentation/themebuilder/advanced-tab