There is no way to change color in LibreWolf, because changing -moz-visitedhyperlink in it is blocked because of fingerprint resistance.

Problem solved. Unpacked the addon with the archiver, changed the line in mod.css content with:
`:visited, :visited * { color: -moz-visitedhyperlinktext !important;}`
To:
`:visited, :visited * { color: #7a7a7a !important;}`
Changed the manifest.json of the addon. Packed the addon. Turned off signature checking in your browser. Installed addon.