It is indeed a slippery slope. Graphics and textures are stored locally on our PCs so are tempting to mod.
However, those assets are all owned by Blizzard and touching any of them without Blizz approval is a violation of the EULA.
In WoW they do ban for using programs to modify graphics, even if only YOU see that graphic.
There are legit or fun reasons people want to mod the graphics - accessibility is one. Fun and amusing things for personal taste is another. However…there are negatives. Making changes to models can cause them to be extra large or small in ways that can give an advantage over other players in fights, esp raids or PvP.
So yeah, Blizz bans for third party image editing in use with WoW.
I would say to stay on the safe side, don’t think you can mod things that connect to the Bnet servers. IF that changes, be surprised.
The forum software is called Discourse. You can learn about it on the website for the software vendor. Blizz has a modified version of it.
There is a pinned post on D3 General on how the basics of the Trust system works, but it is old and they customized the requirements for the Diablo forums to match the traffic flow patterns here. So they are a bit easier to achieve than they were when the post was made. New Forum Guide - Syntax & Features