tests: switch to a dark theme
tests: switch to a dark theme

Personal preference that these tests run under a dark theme rather
than a bright white theme.

Change-Id: Ied043f00024d26b3e70c40764f818f41df71d8d1
Reviewed-on: https://chromium-review.googlesource.com/c/1390220
Reviewed-by: Vitaliy Shipitsyn <6f51537d61e46a00bf958b60dbef5b5870c2bf22@google.com>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

chromium/hterm,chromium/hterm,chromium/hterm,chromium/hterm