[Epic] Benchmark performance gains of CSS variables for theming/color mode

[acstll]

✨ Summary

Assess whether leveraging CSS variables for theming could represent significant performance gains, namely in Kibana.

❗ Problem or Opportunity

Handling of theming, color mode, etc in EUI is handled with JavaScript logic. A change means computing a new theme and updating the styles of each and every component on the page. We can let the browser do this, and remove a decent amount of code/logic.

🎯 Value

Performance and DX (less code).

🎯 Urgency

This is inline with the current efforts to improve perceived performance related to how we write and use Emotion. It'll be enabling the actual implementation if there are actual performance gains.

🧠 Feasibility & Direction

We want to:

• make a PoC that handles theming with CSS variables
• benchmark performance in Kibana

❓ Open Questions to answer

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