Apply Dark Mode Colors to WebAPK Shell

Apply the dark theme color to the app status bar and the dark background
color to the app splash screen.

Fixed: 1467590
Change-Id: I1a1d072a3a93cf2c0995c824bdc68af023dc1054
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4718305
Reviewed-by: Ella Ge <eirage@chromium.org>
Commit-Queue: John-Wes Appleton <johnwes@google.com>
Code-Coverage: Findit <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1178546}