Searching for and downloading Moodle themes is a great way of making your Moodle site look different than the standard pre-installed themes. There are literally thousands of Moodle themes available on the Internet, if you know where to look.
As with pre-installed Moodle themes, many of the themes that you can download from the Internet are free, and all of them are also open source. This means that you can usually download them freely and change them to suit your needs. However, it is worth noting that open source doesn't usually mean that there aren't any restrictions or copyrights placed on the work. Most open source software or code available on the Internet is copyrighted and does normally have some sort of restriction on use. You may be required to leave the copyright credits in the code (attribution) to use only what you have downloaded for non-commercial purposes. Attribution is the concept of requiring the author to be credited, so,...