Ok  - Thanks. 

Hi, it's bug, we will fix in nearest update

Do you know how to change color of the registration page backgrround ? On the desktop is black, but on mobile it turns white. 

Done - thank you.

https://rehubdocs.wpsoul.com/docs/rehub-theme/basic-settings/setting-menus/#How_to_hide_some_menu_items_on_mobile_or_desktop

Unfortunately you didn't attach link so i am not able to test. You can find header options in theme options - header option and you can enable/disable there icons

I have added a link to the registration page to the Primary Menu as a Custom Link. Question is how can I disable it in desktop version? In desktop version there is login icon on the right side. Another "Log In" link is now redundant as it is repeated.

There is no such thing like mobile menu. There is main menu which also used
in mobiles. You can add link to page with your registration form in
Appearance-menus

I installed Buddypress and created a registration page. How can I add LOGIN/SIGN UP option to Mobile Menu? 

You asked specific question how to add social login on site and I sent you
link to direct specific part of documentation which describes how to do
this in theme. Did you read it? If you read it, did you use plugin? If you
used, did you have a problem?

I asked a specific question and expected a specific answer. If I have to figure everything out on my own then I don't need a link to the documentation. 

We will appreciate that you use search in documentation before requesting support. If you click on link which I sent, you can see first result with direct answer on your question. Do you have problem with it?

Thank you for this useless reply. Basically you can answer any question with this link:

https://rehubdocs.wpsoul.com/

https://rehubdocs.wpsoul.com/?s=Social+login&search_param=all

Hi - I want to create register page and add Fecebook/Google login buttons to this page (and ofcourse Facebook / Google login buttons must also be visible in the login popup). 

How to do this?