* Use `@media (prefers-color-scheme: dark)` instead of `[dark]`.
* Turn on kWebUIDarkMode feature on all platforms - this feature now
  gates the above @media directive.

Chromium change:

https://chromium.googlesource.com/chromium/src/+/6e5a6179e616f62bb575b1cd5e1c078fd4a0bc47

commit 6e5a6179e616f62bb575b1cd5e1c078fd4a0bc47
Author: Dan Beam <dbeam@chromium.org>
Date:   Fri Jun 7 21:44:46 2019 +0000

    WebUI Dark Mode: update ui/webui to use @media queries

    This changes the implementation around how dark mode is queried in CSS.

    Bug: 965811

https://chromium.googlesource.com/chromium/src/+/f0a7e117061208634ebf1cff20c4253db53f1b7a

commit f0a7e117061208634ebf1cff20c4253db53f1b7a
Author: Dan Beam <dbeam@chromium.org>
Date:   Fri Jun 7 18:40:58 2019 +0000

    Make @media (prefers-color-scheme:) respect features::kWebUIDarkMode
    while on chrome:// URLs

    This allows us to use this mechanism in CSS:

      @media (prefers-color-scheme: dark) {
      }

    to vary styles for Chrome web UI in dark mode while also disabling via
    feature on certain platforms.

    This unblocks us from migrating to the @media query (from the existing
    [dark] attribute syntax) and later removing a bit of JS/C++ custom code
    in favor of simply using the web platform.

    R=futhark@chromium.org
    BUG=965811

https://bugs.chromium.org/p/chromium/issues/detail?id=965811