I tried to change the "Top Logo" and the "Login Page Logo" by following your steps, but that did not work for me.
After restarting Kibana, I see no change at all. It does not matter if I even make a mistake while editing the code. It looks like Kibana just ignores the changes I made. When I inspect the code from the browser dev tool, none of the changes I made are there, but just the original code. Maybe I am editing the wrong files or maybe there is something else double checking the code and overwriting it at execution time like boostrap. Every time I change the code, I clean the browser cache and all local stored data before testing.
I know there's an official project in progress to add a feature that will allow users like me to easily customize Kibana UI. I am very exited about it and I can't wait to see it completed, but in the meantime, I'm OK with just temporary workarounds that allow me customize the UI. So if you have any more tricks up your sleeve, I'd be happy to try them out.