.td-box--white should be dark-mode compatible

[chalin]

• Context:
  • Dark mode open-telemetry/opentelemetry.io#4023 (comment)
  • Dark mode open-telemetry/opentelemetry.io#4023 (comment)
• Contributes to Support dark mode #331

Tasks

• Dark-mode override for .td-box--white #1966
• Restrict .td-box--white style changes to dark mode #1968