Partial answers appreciated, I got a big field of ignorance to address...

I have a moderately ancient distribution, slackware 10, which came with
xfce4 4.2.1 as the default desktop environment. I've always been a bit confused by X, and I'm not sure whether xfce is a windows manager, a desktop environment or both. And I don't know what files to look in to find out.

But anyway, at xfce-look there are screenshots of all these cool looking desktops, and I can't figure out how to download and use the tehmes files there.

A lot of apps give me messages like

(xfce4-about:3701): Gtk-WARNING **: Unable to locate theme engine in module_path: "xfce",


And I don't know how to fix that.

And clicking around in the xfce settings, I see "theme" is used in multiple ways - I can chose "themes" in a desktop utility called "setting/user interface settings" various themes, but not of them look so radically different as at the web site above.

Xfce 4.2 is old, most themes up there now were made for 4.4, so I don't know how well they would work, if at all. There were major changes to Xfce between 4.2 and 4.4.