if you just want to change the background color just make sure that your css selector is more specific or as I mentioned before jQuery Colorbox will allow you to change the whole theme.

I have not used WordPress HTTPS but there is no need (as you can see xparkmedia.com is running on SSL)

add these 2 lines of code to your wp-config.php file

define('FORCE_SSL_LOGIN', true);
define('FORCE_SSL_ADMIN', true);

and this code to your .htaccess file

RewriteEngine On
RewriteCond %{SERVER_PORT} 80
RewriteRule ^(.*)$ https://DOMAIN.com/$1 [R=301,L]