I miss Dark Reader, UBO is pre-installed, but the only way I can see to install Dark Reader is through firefox add-ons page. I’m hoping that simply going to that page to install the add-on is relatively safe, I wouldn’t feel too worried visiting the mozilla website generally, but given I’m installing extensions the browser I wonder if I’m somehow undoing the good that’s done switching to LibreWolf.
IIRC Dark Reader is discouraged in general from a privacy perspective due to it making you appear more unique. If you’re using LibreWolf primarily to improve your browsing privacy, it’s worth considering whether you still need Dark Reader.
However I don’t believe there are any negative security implications with installing it from a.m.o and using it in LibreWolf. Just remember that for each extension you install, you have to extend the level of trust you grant your browser to cover each extension developer too.