After changing from UEFI to legacy, you may also need to turn off secure boot:
https://www.top-password.com/blog/set-windows-8-pc-to-boot-with-legacy-bios-mode-instead-of-uefi-mode/
Hope you can get the CD booted.
Download the freeware "Windows 10 Login Screen Background Changer", it lets you change the login screen background image:
http://www.top-password.com/blog/change-background-image-for-windows-10-login-screen
This method works for me.
It's worth a try.