As specified in https://www.w3.org/TR/css-ui-4/#appearance-switching.
<rdar://problem/83640938>
Created attachment 439532 [details] For EWS
Created attachment 439620 [details] Patch
Committed r283269 (242298@main): <https://commits.webkit.org/242298@main> All reviewed patches have been landed. Closing bug and clearing flags on attachment 439620 [details].