In addition to the built-in skins of Smart Keyboard, you can also install external skins on your phone to customize even more your keyboard!
There are two options to get new skins:
Open Skin format
This is a skin format developed specifically for Smart Keyboard, but as this format is open, it can be used by other keyboards. An open skin is a simple .zip file (you don't need to install an app from the Android Market), which you can copy on your SD card (in /sdcard/skins folder), and it will appear automatically in the skin list in Smart Keyboard Pro settings (see this page for further instructions).
You can find and download a lot of skins made by Smart Keyboard users on the Open Skins forum.
Here are some popular skins you will find on the forum:
Better Keyboard skin format
Smart Keyboard also supports the skin format developed for the application "Better Keyboard". It means you can get skins for Better Keyboard on the Android Market, and they will also appear automatically in the skin list. Note that some skins (all the ones developed by "Better Android") are read-protected, so cannot be used with Smart Keyboard and will not appear in the list.