There are thousands of GTK+ and Metacity themes available to change the look of Ubuntu, and to install them it’s as easy as dragging the archive file that contains it (usually a .tar.gz or .tar.bz2 file) onto the Appearance dialogue (System > Preferences). But while some will install just fine, they won’t actually look as intended because the “engine” required to render the graphics is not installed. If you’ve seen screenshots of the theme, this will become apparent immediately, as smooth gradients and glass-like effects will not be possible without the required engine.
You can check your installed themes for missing engines by clicking the Customise… button and selecting the theme under Controls. If an engine needs to be installed, it will tell you which one beneath the list of themes.
To look for engines, open Synaptic and search for gtk2- and browse through those packages with “engines” in the name.
Obviously, you can just search for “aurora” or whatever engine it is you’re after, but you may as well save yourself doing this again later, especially if you plan to install a variety of the latest themes.
Mark the one you needed for installation, as well as any others you think might be needed down the line. Since they are small downloads and won’t conflict with one another, you may as well install a bunch of them while you’re there. Engines of note include Aurora, Murrine, Nodoka and CleanIce.
You’ll occasionally come across engines that aren’t in the standard repos (like “ubuntulooks“), so just do a web search for the name and you should find the download page.
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