Form error text in dark mode has low contrast

Problem/Motivation

Originally reported by @smustgrave: #3604037-28: Indicate that grouping elements have child element errors for UX and a11y.

In dark mode, labels for invalid form fields have too low contrast (4.47:1). Results on WebAIM Contrast Checker.

WCAG: SC 1.4.3 Contrast (Minimum) (Level AA)

Steps to reproduce

1. Standard installation.
2. Use dark mode (set browser or system preferences to prefer dark, or emulate prefers-color-scheme: dark in devtools).
3. Go to /user/login while logged out. This should be a login form.
4. Click the Log in button without entering any username / password.
   This should create form validation errors and red labels for the invalid fields.
5. Run a scan with the Axe DevTools or Accessibility Insights browser extensions.

Expected

All red error text has contrast ratio of 4.5:1.

Actual

The red field labels fail color contrast checks.

Proposed resolution

• Use a color for error form labels that has a contrast ratio of at least 4.5:1 against the background.
  Suggested: #d67a7a.
• IMO, use the same color for field borders, etc for consistency.
  Field borders do pass with the current color because non-text contrast requires a lower ratio, but consistency would probably be desirable.

Here's a before / after screenshot from #ce6060 to#d67a7a.

It has 4.71:1 contrast on the WebAIM Contrast Checker and passes the browser extension test.

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet