CSS/JS: Updated build system and theme selection

[pglombardo]

This PR updates the previously outdated bootstrap system that was in place.

Included is the latest themes from the bootstrap project as well.

This PR shouldn't introduce any changes (besides the theme updates) for users but instead area all internal updates/improvements.

Description

Related Issue

#1454
#1377

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix

Checklist

• I've written tests (if applicable) for all new methods and classes that I created. (rake test)
• I've added documentation as necessary so users can easily use and understand this feature/fix.