Well thats because as soon as you search you are actually using a website. If the website doesnt support darkmode or was someday configured to be whitemode, then it will always show up bright. If the website doesnt support darkmodes: use the newest install that comes in a few days the 120 version. Or switch to the nightly-installation where you can already now install plugins/extentions. Then install the darkreader extention. Done. Ezpz
This is the way. The dude in the comment above the one I am replying to described a different way where you have to change the config, which would be neccesary to do on every new installation. Via the bookmarks/shortcuts method you will have the BANG Search on every new installation just by importing your bookmarks - or if you use an firefox account with synchronisation you will already have it with your login.
Be advised that duckduckgo has already some predefined BANG Searches, that you can use without having to change anything. Just type in the search bar:
Well thats weird. Maybe try the nightly build, they have additional features and patches. Maybe its a weird bug that got patched with this version or it is a random problem that occurred whilst the installation.