Derpibooru is extremely hostile in their website design. They use jQuery as a way to confuse just what a piece of code is doing, and also obfuscate all their code scrambling variable names, removing whitespace, etc. Their code has no interface, and exposes no (legible) variables, as much as possible out of reach of any userscript (or other script for that matter).
The vast majority of all the code of their website does nothing but attempt to confuse anyone as to what the rest of the code actually does. This allows them to, for instance, disable the downvote button for people not willing to view the thumbnails of diaper fetish pictures, and show a message chiding you for attempting to downvote it. They try really hard to make sure nobody is in-the-know enough to be able to change that.
They regularly change their code too, randomizing all the obfuscated variable names, and rearranging everything possible into a different order. That's why you see a link to application-8ee2b09edeb44db21308d5ee05295a26.js and not just application.js.
They're one of the hardest sites to write userscripts for. Whoever is coding the thing should be dragged out into the street and shot. Along with whoever is programming Google Docs.
Not saying I couldn't, but it probably wouldn't keep.