Fix #927: improved title contrast on landing page with clear background

Closes #927 (closed)

Eventhough the text isn't supposed to be displayed on a completely white background, this improves the constrat. Title should also occupy less spaces on narrower devices.

cc @funkwhale/reviewers-front @silksow

Try it live: http://funkwhale.pages.funkwhale.audio/-/funkwhale/-/jobs/30859/artifacts/front-review/?_server=https%3A%2F%2Fopen.audio (replace the urle after _server with your own pod url)

Before (contrast)

After (contrast)