[EUIBottomBar] Contrast in different themes

[patpascal]

Problem Statement

We noticed that EUIBottomBar's high-contrast appearance doesn't translate well in Dark Theme. We also feel the contrast could be softened a bit, so we were wondering if this is something you'd want to review. I'd add that keeping always a dark scheme for it makes it feel like a console bar, which is far from the intended usage.

Proposed Solution

We explored the current situation + an alternative, in case it helps. Our preference would be the 3rd solution, finding a mid point that works well for both themes without being too harsh

Use Case

All around Streams (Advanced, Partitioning, Processing, etc... it's our submit queued changes mechanism), Management Advanced Settings, etc.

Urgency

Not a blocker, but a nice to have

Do alternatives or workarounds exist?

No other than customizing the component but that would break the pattern.

Designs or Specs (Optional) Explorations

Discussion: https://github.com/elastic/eui/discussions/9203