Disable dark theme in until chat history has dark theme support

[compulim]

Fixes #4863.

Changelog Entry

Fixed

• Fixes #4863. Disable dark theme for link references until chat history has dark theme support, by @compulim, in PR #4864

Description

Temporarily disable dark theme until chat history support dark theme.

Otherwise, link references will look weird.

And link texts in modal dialog will also look weird.

Design

• DARK_THEME_SELECTOR will always falsy (select nothing)
• LIGHT_THEME_SELECTOR will be truthy if not in forced colors

Specific Changes

• Updates Constants to disable dark theme selector

• I have added tests and executed them locally
• I have updated CHANGELOG.md
• I have updated documentation

Review Checklist

This section is for contributors to review your work.

• Accessibility reviewed (tab order, content readability, alt text, color contrast)
• Browser and platform compatibilities reviewed
• CSS styles reviewed (minimal rules, no z-index)
• Documents reviewed (docs, samples, live demo)
• Internationalization reviewed (strings, unit formatting)
• package.json and package-lock.json reviewed
• Security reviewed (no data URIs, check for nonce leak)
• Tests reviewed (coverage, legitimacy)