Hello. The Pro Sites plugin by WPMUDEV generates the checkout page where customers pay to activate their website. It's a premium GPL plugin.
I've deactivated all plugins, and the CSS still doesn't load, so I think it's an issue with the Pro Sites plugin.
This is how the slylesheet loads:
<link href='//domain.com/wp-content/uploads/am_assets/head-1fdba110e2a0286224d5641310cd3c0b.css' media='screen, projection' rel='stylesheet' type='text/css'>
When I click the link to the stylesheet, it loads fine and I can view it, so it works. The CSS for this particular page is included in the combined CSS file as well, it just isn't recognized by the page for some reason.
Email me at gm7 (at) charter.net and I can send the plugin files if you want to try to find what's wrong. Even though it's GPL, I prefer not to post it publicly. If it turns out there is a bug in the plugin, I can submit it to their support team for a fix. If it turns out that it's just this one plugin, I understand if it doesn't make sense to spend time on your end fixing it, no worries.