I’m also on a 20.04 install and get the branded boot screen and I really like it, especially it is black and not purple. 
I agree that the spinner should be the same systemwide and I do also agree with some comments here, that the gdm background should be changed. The purple there is just plain ugly and a dark grey from the yaru theme would fit so much better with the new boot splash.
One question: I know how I can change the background color in gdm but there is still a purple splash after login right before gnome-shell loads. Anyone knows how I get rid of that?