Gerrit 3.4 deprecates HTML-based plugins, so the old theme doesn't
work. I have reworked this into a javascript plugin.
This should look the same, although I've achieved things in different
ways.
This doesn't register light and dark variants; since
background-primary-color is white, by setting the
header-background-color to this we get white behind the header bar,
and it correctly switches to the default black(ish) when in dark mode
(currently its seems the header doesn't obey dark mode, so this is an
improvement).
I'm not sure what's going on with the extant header-border-image which
is a linear gradient all of the same color. I modified this down to
1px (same as default) and made it fade in-and-out of the logo colour,
just for fun.
Change-Id: Ia2e32731c1cfe97639de2ec0e7660c7ed583e045
6e232716e7