GH-2506 Adapt PUI to user preferred color scheme

[brianzelip]

This PR adds a dark mode theme to the PUI for users who set a dark mode preference at their browser or OS level.

Main changes

• PUI now adapts to user preference for dark/light mode at their operating system or browser level settings
• Refactored all Sass color variables (that aren't Bootstrap Sass overrides in PUI variables.scss) into CSS custom properties
• Added prefers-color-scheme media query to colors.scss which defines dark theme primitives, base, and specific UI colors
  • Color primitives are scale-based and are only directly used from within colors.scss
  • Other files reference dark theme base and specific UI variables for easy institutional rebranding
• Added *-dark.scss files for dark mode specific styles (aspace-dark.scss, infinite-scroll-dark.scss, etc)
• Added Bootstrap-Dark v4.6.1 and customized it to meet AS needs, including:
  • Removed CSS that doesn't apply
  • Disabled and added prefixes to their custom properties to avoid name conflicts if we ever need to extend Bootstrap-Dark

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Related JIRA Ticket or GitHub Issue

#2506

How Has This Been Tested?

Cross-browser visual inspection.

Screenshots (if appropriate):

See above.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have read the CONTRIBUTING document.
• I have authority to submit this code.
• I have added tests to cover my changes.
• All new and existing tests passed.

[brianzelip]

@quoideneuf This is now ready for review!

[brianzelip]

This PR will be relevant to future work. Converting this to DRAFT status for sake of cleaning up PRs summer 2023.