Revert "WebUI Dark Mode: force user manager and print preview into light mode"

This reverts commit 340e60088b1bd792c87f09b483a69dad24ec5b3b.

Reason for revert: this was just on the off chance WebUI Dark Mode made it in M73.  It's probably not going to, but either way: no longer need this temporary change in.

Original change's description:
> WebUI Dark Mode: force user manager and print preview into light mode
> 
> This is in the unlikely event that dark mode actually makes it in 73.
> These UIs are not ready yet.
> 
> R=​dpapad@chromium.org
> BUG=883049
> 
> Change-Id: Iaa79feadd8d341c4b730a9a6598f42c756070613
> Reviewed-on: https://chromium-review.googlesource.com/c/1435517
> Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
> Commit-Queue: Demetrios Papadopoulos <dpapad@chromium.org>
> Auto-Submit: Dan Beam (slow until 1/29) <dbeam@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#625843}

TBR=dbeam@chromium.org,dpapad@chromium.org

# Not skipping CQ checks because original CL landed > 1 day ago.

Bug: 883049
Change-Id: I31278716430d53b1181de4da5044d158aaf33473
Reviewed-on: https://chromium-review.googlesource.com/c/1437746
Reviewed-by: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Commit-Queue: Dan Beam (slow until 1/29) <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626305}
4 files changed