I've always been fascinated with client customization, especially regarding UI. However, there's not a lot out there that allows anyone to fully take advantage of being able to customize the look of the client. What does exist is severely outdated, not even being edited since 2012 in some cases.
[Tutorial] How to edit GUI (2012)
Creating New Themes (2013)
Neither of these posts have been updated, and in the case of the latter, no questions on the post have been answered. To this day, it's still relatively confusing on how to customize the UI in PokeMMO, especially regarding the XML files.
Basic customization allows editing pre-existing images, which is fine, but when you want to move elements around (like on the trainer card) or even create new elements, it becomes impossible. Ideally, some form of how the developers for PokeMMO would explain (and maybe even provide) the tools they use to create UI elements, especially with XML files. If the documentation for customization is more accessible and open, then the customization scene wouldn't be filled with old, outdated themes and the community would benefit greatly from this.