Remove duplicate shades of gray and black

app/assets/stylesheets/framework/buttons.scss

app/assets/stylesheets/framework/dropdowns.scss

app/assets/stylesheets/framework/forms.scss

app/assets/stylesheets/framework/header.scss

app/assets/stylesheets/framework/nav.scss

app/assets/stylesheets/framework/page-header.scss

app/assets/stylesheets/framework/variables.scss

app/assets/stylesheets/framework/zen.scss

app/assets/stylesheets/pages/boards.scss

app/assets/stylesheets/pages/builds.scss

app/assets/stylesheets/pages/commits.scss

app/assets/stylesheets/pages/environments.scss

app/assets/stylesheets/pages/issuable.scss

app/assets/stylesheets/pages/merge_requests.scss

app/assets/stylesheets/pages/milestone.scss

app/assets/stylesheets/pages/pipelines.scss

app/assets/stylesheets/pages/settings.scss

app/assets/stylesheets/pages/status.scss