Some improvements

- Adding documentation
 - Note the effect empty array
 - Fix spelling
 - Using new test `[[...]]`

README.markdown

@@ -88,6 +88,14 @@ If you're feeling feisty, you can let the computer select one randomly for you e
 ZSH_THEME="random" # (...please let it be pie... please be some pie..)
 ```
 
+And if you want to pick random theme from a list of your favorite themes:
+
+```shell
+ZSH_THEM_RANDOM_CANDIDATES=(
+  "robbyrussell"
+  "agnoster"
+)
+```
 
 ## Advanced Topics