DOP-4995: Dark Mode UAT: always on scroll, version selector hover flash, clear all filters flash

[mmeigs]

Stories/Links:

DOP-4995, DOP-4997, DOP-4998

Current Behavior:

Cloud-docs (user always on scrollbar)
Landing search (search filter button flash)
Manual (version selector hover)

Staging Links:

Cloud-docs (user always on scrollbar)
Landing search (search filter button flash)
Manual (version selector hover)

Notes:

These three tickets are from UAT bug reporting spreadsheet.

1. If a user has Always-On scrolling (user setting computer-wide), that particular scrollbar was not in dark mode. This fixes that. Photos below.
2. Clear all filters button in search had a light mode flash on load.
3. Version Selector in sidenav had flash of light mode on load only when on hover.

Scroll photos

Before:

After:

README updates

• • This PR introduces changes that should be reflected in the README, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README

[seungpark]

this LGTM. i'll have to merge this into feature branch as well!