Message search result text too dark with dark theme (Vue left-menu)

Message search result text too dark with dark theme (Vue left-menu).

Also the hover should be dark

Testing locally in dev

This is a bit hard to test locally because message search doesn't work, https://gitlab.com/gitlab-org/gitter/webapp/issues/1919

But we can mock the response, see https://gitlab.com/gitlab-org/gitter/webapp/merge_requests/1411#testing