Wraps the new InvokePluginTheme RPC and ships an end-to-end
test that loads studio-theme-solarized into a fresh
HubFixture and asserts the light/dark schemes round-trip.
Signed-off-by: flemming-it <sf@flemming.it>
Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>
