The theme color is used on android to style the chrome
browser-ui with a color that suits the webpage.
Use the dark brand color instead of a random default blue color
in order to fit to the rest of the color scheme.
Those were added in b41a3f66c9 without
an explicit license, so they are under the default license. Some files
already got a header previously, so this only touches the remaining.
The same should be done for docs/_data/extra_pages.json, but it's json, and
json doesn't allow comments.
