Lets see...in includes/functions.php find: "image_file_size" => $file_size,
Insert below: "noaccess" => $show_link ? 0 : 1,
Now you can add the style you want in thumbnail_bit.html template between {if noaccess} and {endif noaccess} tags
i.e. {if noaccess}style="background-color: red;"{endif noaccess}