The process for installing themes is basically the same as for modules and blocks with, again, the difference being in the location that the package is installed. In the case of some plugins, there may be a need to make minor, or in the case of some contributed plugins, even major modifications to files in the Moodle directory. If the changes are minor and you are comfortable experimenting, then give it a try. This is why you are using a test site right! Some plugins however, require a patch, which is a modification to a core Moodle file or files. In addition to being undesirable, because it makes upgrading to a new version of Moodle more difficult, it can also be technically demanding. You can determine whether a plugin is a patch or not and also determine the installation requirements by viewing its entry in the Modules and plugins database found on the Moodle.org website. If the plugin requires a major patch or some other type of manual modification of code, and you are not a professional programmer, please proceed with caution. Make sure you have tested everything thoroughly on your experimental site before moving it over to your live server.