Define new mode to capture dark mode vision

Purpose

We have explored how to apply our color constants to achieve a high-quality dark mode at both the component and page level. We need to produce a single mode / theme out of this research to add to our SSOT – gitlab-ui's token JSON files.

Proposal

• Update the design tokens spreadsheet to capture the new dark mode vision
• Capture definitions as a new column ("Value Nacht"?)
• Only update tokens on the semantic and contextual tabs
• These definitions should only reference existing constant tokens