Ugly lock screen in Xubuntu

Remove xscreensaver and install gnome-screensaver which has a more polished look:

sudo apt-get remove xscreensaver
sudo apt-get install gnome-screensaver

You can also set a shortcut to lock the screen with the keyboard:

Start > Settings Manager > Keyboard > Application Shortcuts > Add

Command: xflock4

ShortCut: <Super>l ; (e.g. press the "Windows key" + l)

^{NOTE: The only potential downside to this is that gnome-screensaver will pull the gnome dependencies which in a very minimalistic desktop might not already be present. But on most average systems the gnome libraries are already installed anyway so there is no overhead.}

Xscreensaver is a package which is an extensible screen saver framework, and locks the computer.

According to this linuxfromscratch.org page.

The XScreenSaver is a modular screen saver and locker for the X Window System. It is highly customizable and allows the use of any program that can draw on the root window as a display mode. The purpose of XScreenSaver is to display pretty pictures on your screen when it is not in use, in keeping with the philosophy that unattended monitors should always be doing something interesting, just like they do in the movies. However, XScreenSaver can also be used as a screen locker, to prevent others from using your terminal while you are away.

I don't know the reason why it is used in Lubuntu and Xubuntu, but it might be because of the its popularity, stability, and probably it is light weight.

It also reduces the burden of building a new package to replace already existing software.

Do you like black? Try slock, it's the best example of the KISS principle.

To install it: sudo apt-get install suckless-tools

Then, the easiest way to configure it in xubuntu is: sudo apt-get remove xscreensaver

This is because by default xscreensaver takes precedence over other screensavers, slock among them. Once removed, you can either kill the xscreensaver thread already running in background or just restart.

Enjoy simplicity.