In order to add a new social icon in the header, please open room/theme-options/theme.options.php file and add the following code on line 170:

array( 'icon' => 'icon-instagramm', 'url' => ''),

Here is the screenshot:

'icon-instagramm' can be replaced with any other social icon. The list of all theme icons can be seen at the following link: http://room.axiomthemes.com/wp-content/themes/room/css/fontello-all/demo.html

Then you will be able to add the link in Appearance -> Customize -> Socials: 

The icon will show up as soon as you save  the changes.