Outline as form field signifier
How to use it
Add an outline around a form field.
When to use it
If your field has a low contrast with the background colour, a stroke can help to create a contrasting border, which means you don't need to change either the foreground or background colour. A border also looks similar to what you'd find on a lot of paper forms, so it might help people realise that the element is a field.
What to watch out for
If the border is too heavy, either in thickness or the colour is very dark, it might be visually distracting.