Hide the "Already have a user account" link on the Register My Account page

Hello. I want to hide the "Already have a user account" link on the Register My Account page. I found a support forum post that explains how to do this:

https://premium.wpmudev.org/forums/topic/membership-how-to-change-link-already-have-a-user-account

Essentially the answer says to download this plugin:
http://wordpress.org/plugins/simple-custom-css/

And add this line of code:
div#membership-wrapper form.form-membership a#login_right { display: none; }

This works amazing.

However, that plugin, Simple Custom CSS, does not work on pages with an SSL certificate. I have my Register for My Account page protected by an SSL certificate, but when I use the Simple Custom CSS plugin to hide the "Already have a user account" link, it results in parts of my page being listed as insecure, thus I do not get the full padlock for the HTTPS.

I know I can manually go into publicforms.css and add that line of code to it, but I absolutely fell in love with the Simple Custom CSS plugin. Do you know how to make it work with a page with SSL?

Thank you!