IMO we should backport these.
@patrickhlauke: there are probably more color and general accessibility issues, but I try to solve the issues in batches. There's one thing that bugs me:
This form does not contain a submit button, which creates issues for those who cannot submit the form using the keyboard. Submit buttons are INPUT elements with type attribute "submit" or "image", or BUTTON elements with type "submit" or omitted/invalid.
For the search form.
Preview: https://deploy-preview-29208--twbs-bootstrap.netlify.com/