A quick userContent.css definition to undo the greying:
/* openstreetmap.org */
@media (prefers-color-scheme: dark) {
#content #map .leaflet-pane .leaflet-tile-container .leaflet-tile, #content #map .mapkey-table-entry td:first-child > * {
filter: brightness(1.0) !important;
}
}
The #content #map selectors I added to mimic openstreetmap dot org’s DOM structure. It might still catch some other websites, but would make no difference unless they use brightness filters too.