With no disrespect intended towards the Lubuntu project, Kubuntu would likely have more pull for getting SDDM themes into the standard repositories than Lubuntu - since they're bigger and have been using SDDM for longer - and pulling new bling from the KDE store is built into KDE's standard settings so they have less incentive to bother than they would otherwise. Not having that infrastructure so that you have to fiddle with things by hand is the "bloat-avoidance" that people are choosing by going with something like Lubuntu.
I can't speak for ml9104, but some caution is warranted when fiddling with a display manager, since it's possible to render yourself unable to log in with careless fiddling. So things like working with a different theme rather than potentially breaking the only theme (and knowing how to change theme from the command line), trying to avoid breaking Qt, testing in a VM, making backups of files you change so you can easily restore them from the command line if your changes break things, and so on, are entirely sensible precautions. Where I might differ, though, is that it's your computer: should you break things beyond your ability to fix them, a reinstall takes about 20 minutes, and you'll have learned not to do whatever it was that you did. It's entirely possible that ml9104 learned a lesson about those files by exactly that method, hence their note of caution: there are definitely things I tried in the beginning (although I've never tried playing with SDDM themes) where the salient lesson was exactly "yeah, don't do that."
I would definitely start with examining how other themes work, though, so you can get a feel for the structure without having to give your own machine such a hard poke.
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