Many of these need to be fixed with the extension converter before SeaMonkey can install them. If SeaMonkey doesn’t allow you to install them, download them instead and then upload them to:
Dark Background and Light Text
Stylem is like Stylish or Stylus, except better because it had a “check for errors” option right there in the editor. I wish I’d known about this sooner; I have a lot of cruft to clear out of my userContent.css file.