Just like the Cinnamon edition, the MATE edition contains a slew of backgrounds you can use for your desktop. If you don't prefer any of those that are installed by default, you can click on Add… and browse to find a picture of your own to use. The following screenshot shows how to select a desktop background:

Also located in the Control Center is a section where you can change the screensaver that appears during a period of inactivity. In this section, you can choose your favorite screensaver, the length of time until it appears, or disable the screensaver altogether. By default, there are only a few screensavers installed, so this aspect of MATE may be a bit underwhelming.

The panel (the bar along the bottom of the screen) is also customizable using applets. To add an applet, first right-click on any empty portion of the panel. On the right-click menu, select Add to Panel... and the Add to Panel window will appear. From there, a list of available applets will appear along with a short description of each. To add one that interests you, click on it and then click on the Add button. You'll notice that the applet will appear on your panel, but likely not where you'd prefer it to be. To move the applet, right-click on it and then click on Move. From the right-click menu, you can also select Remove from Panel to get rid of it.

In the Control Center, there's a section labeled Appearance. There, you can customize your theme. By default, only the Mint-X theme is installed. Install the mate-themes package, which will add several more themes for you to choose from. If you wish to install more, consider searching on Google for Mate Themes, and you may find additional themes with installation instructions. The following screenshot shows MATE's Appearance Preferences window with the mate-themes package installed:

Once you apply a new theme, you'll notice that the appearance of your panel, window borders, and button controls will completely change.