Add initial (non-functional) UA styles and preference.
<rdar://problem/105311363>
Pull request: https://github.com/WebKit/WebKit/pull/9968
Committed 260160@main (d4485d2afa7b): <https://commits.webkit.org/260160@main> Reviewed commits have been landed. Closing PR #9968 and removing active labels.
Re-opening for pull request https://github.com/WebKit/WebKit/pull/10018
Test gardening commit 260181@main (aeb86bf80138): <https://commits.webkit.org/260181@main> Reviewed commits have been landed. Closing PR #10018 and removing active labels.