@Slinky_McVelvet, I think you are a bit confused here. Joey has stated it above, but as an extremely grateful user of the fine work the themers on this board have done, I would like to clarify as well for you and anyone else who reads the thread or has the same question.
You have two very different things here.
1) One is theming, or modifying the user interface, which is freely allowed by WD.
2)The other is modifying the Firmware, which is changing the internal behavior of the device itself. WD states that will void your warranty.
The hundreds or probably thousands of us, who have simply installed themes on our WD Devices, have added modified code to our boxes that altered the user interface. This is fine, and WD has even pushed the fact that coders could write new themes for the devices by modifying the code for the UI (the XMLs and the JPGs), and then share them. You will notice that these themes are freely distributed on these boards.
FW modification is a completely different thing. You know when you are updating your Firmware as you are asked if you would like to upgrade the version and then the device goes through the UPGRADE process. This does not happen when you add a theme. You install it, and then you have a modified user experience.
WD’s warning state that if you use the GPL code to UPDATE the device, it will be treated as third party firmware. Luckily for you, all the incredible and generous coders here, as well as the slews of folks who have installed the great themes on this board, you are not updating the device. You are just changing the front end with the XMLs and some of the graphics.
I am assuming you have done a Firmware upgrade on the box when you bought it to get to the most current version. If so, then you know the difference between the two. A modified FW upgrade comes via a thumbdrive, and adds / removes options, apps and modifies the entire behavior of the product. During the process the device will tell you the percent updated and then reboot multiple times. Once completed you can then go to the configuration menu and see that you are now running a newer FW version. A theme just alters the way you view and access that firmware backend. It does not require a reboot, and when you look at the configuration menu and go to check the FW version, you will find it exactly the same as where you were at before.
Joey has provided the code that is JUST FOR THE INTERFACE (THEMES), so is completely allowed for modification. When uploaded via the web interface on the SMP, there is no way to alter the FW. That mechanism is built specifically to upload theme code to your SMP.
I hope that helps make this clear for you. I am now and have always been super grateful to the amazing coders on this board. Their generosity and creativity has made these devices truly customizable and enhanced all of our experiences. The code work that Joey and others have done has not in any way voided their or any of our warranties, it has only enriched the product for the rest of the end-users here.
Best of luck, and a highly encourage you to keep your SMP and modify the XMLs to alter the interface in a way that suits your needs. I only hope that you will share your work with the rest of us!