Adjust lowlight link alt to show use on dark and light backgrounds. [Finishes #80243932]
Adjust lowlight link alt to show use on dark and light backgrounds.
[Finishes #80243932]

Signed-off-by: Josh Franklin <acbc767a39669c3acb55c05fcee4aa904747a876@pivotal.io>

npm/pivotal-ui,npm/pivotal-ui,sjolicoeur/pivotal-ui,CanopyCloud/pivotal-ui,pivotal-cf/pivotal-ui,CanopyCloud/pivotal-ui,npm/pivotal-ui,CanopyCloud/pivotal-ui,pivotal-cf/pivotal-ui,sjolicoeur/pivotal-ui,sjolicoeur/pivotal-ui,sjolicoeur/pivotal-ui,pivotal-cf/pivotal-ui