Dark Mode: fix Extensions page dev mode toolbar and loading shell

Changes the background color of the "dev mode" bar to match spec (GM2
GG800) and removes bottom border (not in spec mocks).

Also changes the loading shell to match.

Screenshots: https://imgur.com/a/nbQBRik
Spec: https://docs.google.com/presentation/d/1kJoBzf_HGYK-_FAJPAjD67TW8224dCPLA6dAhlDmysk/edit?ts=5c363f50#slide=id.g481b97d74a_0_359

R=dpapad@chromium.org
BUG=883049

Change-Id: I16aef0496775fd9b6b678085d26077c6525483a1
Reviewed-on: https://chromium-review.googlesource.com/c/1426158
Reviewed-by: Demetrios Papadopoulos <dpapad@chromium.org>
Auto-Submit: Dan Beam <dbeam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#624870}
3 files changed