S2 Members Plugin - WordPress - Login Page Not Loading Stylesheet

I've inherited a site to finish and launch from my colleagues who are unavailable to help me figure out where things left off.

It's a WordPress site running on a multi-site network (WP Engine host). I'm using the S2 Members (Pro) plugin. Everything looks great, except for the Login Welcome Page, which is not loading stylesheets. I'm able to customize certain things with Theme Options, but the font styles are not loading, even when I add custom CSS to those fields in the Theme Options and the Page Options.

I see this article with a similar question: How do I secure a stylesheet?

I believe this is related to the 'https' on the page, and I see the recommendation to change the URL to a relative path.

However, I'm not sure where to do that. Can anyone point me in the right direction? Unfortunately the site is locked down to the public, so I can't post the URL for you to see. But this is the error I'm getting in the console:

Mixed Content: The page at 'http://ift.tt/19ZBgRp' was loaded over HTTPS,
but requested an insecure stylesheet 'http://ift.tt/1TUpLPr'. This request has been blocked; the content must be served over HTTPS.

Thanks so much.

via Chebli Mohamed