Connect to the Internet and navigate to Moodle.org, and then browse to About | Demonstration site (in the top menu).

Once you are in the Moodle demonstration site, scroll down to the English category, and click on the Moodle Features Demo link. This will take you to the Moodle features demo course. Moodle might ask you to log in and if it does, then log in as "admin" with the password as "demo". You will then be taken to the Moodle features demo page, and in the top left-hand sideblock, there will be a link to download the Moodle features demo course. If you cannot see this block, then it could have been deleted by other users. The Moodle features demo course is reset every hour on the hour, so if it's not there, you might have to try again after an hour.

Download this demo to your desktop or somewhere that you know where it is. Now go back to the Moodle site and create a new course, and then restore the Moodle features demo to this course. I am assuming that you know how to do this, but if you don't, there are plenty of tutorials and documentation on Moodle.org.

As you can see from the previous screenshot (not the whole course), all of the Moodle features are present here.

Now that you have the Moodle features demo course installed, you can use this to test all of the features of your theme. This is a great way to test, as there is nearly always something that goes wrong in the theming process that could be overlooked if you don't check every feature thoroughly in all of the most popular browsers.

So go ahead and open Internet Explorer, Firefox, Google Chrome, and Apple Safari for Windows and test each feature, one at a time, in all these four web browsers. You are not testing functionality here, just your theme and how it looks for all of the Moodle features.