I just upgraded to 6.0 from 5.0.18. I went into the Admin area, then Global Settings/Themes. I set the logo URL and hit save, then restarted the server. It then rebranded it, but the URL I provided was pointing to an invalid logo (so not the one I wanted). So I tried changing it to another location. Changing the "Application logo banner of the URL..." to empty or something different didn't trigger the "Save" button to light up, thus it NEVER saved it. I tried trickery, changing something on the same page "Reset all color of the themes" to get the save active but it never saved the bottom portion.
I then tried to zmprov flushCache skins from the commandline. It succeeded but still nothing happened. So I ended up with a wrong logo which I can no longer change the reference to.