[PoC] Support dialog theme/color configuration

[st0012]

See https://bugs.ruby-lang.org/issues/19010

Todos

If we want to proceed on this idea, we have the following todos:

• Add test cases
• Release reline 0.4.0 for the new APIs
• Bump reline requirement and drop Ruby 2.5 support because reline now requires 2.6+
  • This includes CI changes
• Add documentation
• Automatic background detection to toggle bright theme

[st0012]

cc @peterzhu2118@nobu@hsbt

[peterzhu2118]

👍 I like having themes. Makes the UX better.