That Login Background Image...

There's a very easy way to avoid that ugly screen: setup the login so that you don't have to enter a password but go straight to your user account.

However, if you're in a location where there are others you would not want to have access to your Mac you'll have to just learn to enter the password and get to your user account as fast as possible.

You can also tell Apple what missing features you'd like restored or new features added to the system via Feedback - macOS - Apple.  If enough complain they may make it user changeable.

There hasn't been a simple way for a long time. Can you clarify where you are seeing this? There are actually quite a few different types of "login screen", each with its own set of complications:

1) FileVault boot screen

2) Non-FileVault boot screen

3) Login screen

4) Screen lock

#3 and #4 are easy to change. #1 and #2 are virtually impossible. In Big Sur, they might be definitively impossible due to security protections on the boot system. Plus, we now have to deal with T2 and M1 machines, both of which change all of the above.

So there is no answer I could give you that wouldn't be extremely difficult, with less than 10% chance of success.

andynick wrote:

It's the first screen I see when I power up the computer, and has the window for a password - the Login Screen, I presume??

I'm not sure. FileVault is special because the hard drive is still locked when that screen is displayed. So any change you might make to customize it will be also be locked and unavailable. I don't know if you are using FileVault or not. And as I said above, newer hardware may complicate this process even further.

FileVault is encryption. This way, if someone steals your computer, they can't get any of your data. FileVault is a good thing. Most people don't stare that login screen for too long. They just login and it goes away. Most people also stay logged in. There really isn't much need to log out.

However, due to the way the boot up process works, it is very difficult to change that initial sequence. If you were able to easily change any of that data, then, in theory, an "attacker" could also do that and change the operating system. Yes, I realize your computer is in a secure location and the chances of this happening are infinitesimally remote. Those facts don't matter.

If it were possible to change the FileVault boot data, there would be headlines on every major media web site and TV show about "MAJOR APPLE SECURITY FLAW" and then Apple would change it and you wouldn't be able to choose your login screen anymore. All Apple is doing is cutting out the middle-man of the bad press and jumping right to the end. This is the world we live in now. We can't have nice things.

andynick wrote:

I like to power my mac down at night...

That really isn't a good idea anymore. You can just let it sleep. It will make backups, perform system maintenance, and download security updates overnight. You can even have it automatically install app updates and system updates. There is even a setting for it to download system updates but not install them. If you then do decide to update, it might save you a few minutes depending on your bandwidth.

Seems I'm stuck because I like to power my mac down at night...

I do the same thing. Aside from the fact that my Macs have always performed better/snappier after booting up (going through some cleaning up at that time), I do not want it to download or update anything in the background or without my knowledge, so all of those options are off. So, I will continue to stare at that questionable taste screaming colors screen and quickly enter my password.

Did you unlock the padlock at the bottom of the preference pane?