Network index should be a non-theme html

Following: https://www.ads-software.com/support/topic/basic-landing-page-not-loading-cssimages/. has been imposible to develop a theme. Because the very basic infraestructure of the landing page. Adding requirements as headears just broke everything.

There is a way to baypass the wordpress-index and load the index from a custome folder/file, while keeping the rest of network working as wordpress is supposed to?