shortcodes to customize registration page in membership plugin not working?

My goal is to have a fully customized registration page via the membership plugin. Here is an explanation of the issue I seem to be facing:

1. I changed the buttons.css file so that the "blue" and "rosy" buttons were completely different colours to match my website, and the I was going to go into other files like the popover.signup.form.php file and change the button colours to those two. But, as was pointed out in another thread, these changes will be erased with every update. I'm not exactly sure how I can change this whithout it being erased. I'm a novice when it comes to php and a beginner (more or less) to css, so I'm not 100% sure how I could change the functions.php file of my theme to accomplish this.

2. I was hoping to completely customize the registration page itself and add my own content, especially when it comes to the price boxes ... I really don't like the price boxes, especially when my theme hides the sign up button. So if I don't change the subscription.form.php file, I'm not sure how else to accomplish this.

I went this route after realizing that changing this file isn't a good fix:

"This is the page a new user will be redirected to when they want to register on your site.
You can include an introduction on the page, for more advanced content around the registration form then you should include the [subscriptionform] shortcode in some location on that page. Alternatively leave the page blank for the standard Membership subscription forms."

This is written in the "membership pages" tab under options. I tried the shortcode and it didn't work. I also made sure that shortcodes were selected and placed as a positive rule for each access level, and I made sure I was not signed in as admin. I noticed though, that the subscriptionform shortcode was not in the shortcode list to select for the access levels, so I suppose that's why it's not working ... because technically it's "protected content" ... I don't know how to fix this, though since it's not on the list for me to select for the access level rules. Any ideas?

Thanks for all your help!!