WebUI Dark Mode: update cr-radio-button colors

Screenshots: https://imgur.com/a/wEShNS0
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g364d7f9db1_0_8

Bug: 883049
Change-Id: I675df4e74c8530de14a3b9dc662c61446a715f6d
Reviewed-on: https://chromium-review.googlesource.com/c/1427741
Commit-Queue: Dan Beam <dbeam@chromium.org>
Commit-Queue: Esmael El-Moslimany <aee@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Reviewed-by: Esmael El-Moslimany <aee@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625014}
1 file changed