Check the permissions on your Ruby script (may not have execute permission), your theme file and directory (in case it can't read the theme or tries to create other themes in there), and the directory you're in when you run the script (in case it makes temporary files in the current directory rather then /tmp).
The sudo command executes the commands which follow it with 'superuser' or 'root' privileges. This should allow you to execute almost anything from the command line. That said, DON'T DO THIS! If you are running a script on your computer and don't need it to access core components of your operating system (I'm guessing you're not since you are invoking the script on something inside your home directory (~/)), then it should be running from your home directory, ie:
Move it to ~/ or a sub directory and execute from there. You should never have permission issues there and there wont be a risk of it accessing or modifying anything critical to your OS.
If you are still having problems you can check the permissions on the file by running ls -l while in the same directory as the ruby script. You will get something like this:
You will notice that the readme.txt file says -rw-r--r-- on the left. This shows the permissions for that file. The 9 characters from the right can be split into groups of 3 characters of 'rwx' (read, write, execute). If I want to add execute rights to this file I would execute chmod 755 readme.txt and that permissions portion would become rwxr-xr-x. I can now execute this file if I want to by running ./readme.txt (./ tells the bash to look in the current directory for the intended command rather that search the $PATH variable).
schluchc alludes to looking at the man page for chmod, do this by running man chmod. This is the best way to get documentation on a given command, man <command>