Dark Mode: update Bookmarks page folder icons and selection colors

Folder icons:

Use -webkit-mask-image instead of background-image allows us to change
the color of the fill dynamically. This means we can create light/dark
with the same .svg.

Selection colors:

Dark mode needs more code for selection (than light mode) because the
specs call for inverting the background to light blue (so the text and
icons must be flipped to dark on light, whereas they're typically light
on dark).

I also strove to generally simplify some border/background color logic
by using currentcolor, which lets other properties use the text color
as a value.

Screenshots: https://imgur.com/a/TDqd9KY
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g481b97d74a_0_342

BUG=883049
R=dpapad@chromium.org

Change-Id: I4ff4ec2d6f29fbdd637e00e2c6818eb26d0b5c1b
Reviewed-on: https://chromium-review.googlesource.com/c/1423701
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624439}
5 files changed