First of all,
Welcome to the site!
Second of all, you're not an idiot. The whole thing can be pretty overwhelming at first but it's actually pretty easy once you get started.
All you have to do is first download desktopx if you haven't already. Here is a link:
Once you've done that, and installed it on your computer, launch the program called "desktopx builder"
Click the button called "load a desktop" to select from some preloaded themes.
to download more themes, just go here:
once you download a theme, you can doubleclick it to launch it with desktopx or you can go to that button again and browse for it on your computer.
Remeber, these themes are made by users just like you or me, so always remember to thank someone for allowing you to download their theme.
hope that helps!
Ps. desktopx documentation, can be found here: