remove default background-color from breadcrumb

[m5o]

• remove default background-color from breadcrumb
• re-use contextual background classes for background styling

[mdo]

Removing variables breaks backward compatibility. We'll want to leave the defaults and add docs for how to override those with utilities. Come v5, we can revisit the default styles.

[m5o]

@mdo UIkit has a .[uk-]text-reset / .[uk-]link-reset util class which reset color and text-decoration. This would be super handy.

a.uk-link-reset, a.uk-link-reset:hover, .uk-link-reset a, .uk-link-reset a:hover {
  color: inherit !important;
  text-decoration: none !important;
}

📖 https://getuikit.com/docs/link#link-reset

[mdo]

Closing this out since it's stale and cannot be merged as-is into v4. Happy to revisit in v5, and to revisit the link reset in a subsequent minor release if there's a PR for it.