I understand your frustration and would suggest you contact your premium theme support. A good support will be able to offer you some CSS modifications even if the issue is brought about by a third party plugin, or if the issue goes beyond the level of support that they normally provide, they will at least be able to guide you in the most civilized way like all paid themes do.
Or if you are code savvy enough, you can consider Henry Wright’s reply to my post. For a free theme support, his reply is useful and polite.
I am running on Divi theme, and there is no way that I can use Divi theme to edit the layout of the registration page because it’s done at the backend of the BuddyPress plugin.
Is there a plugin out there that can resolve this ?