[Console Monaco] Improve color contrast of selection highlighting

[ElenaStoeva]

Closes #186772

Summary

This PR fixes the color contrast issue with the selection highlighting in Console Monaco.

Before:

Now:

Note: I changed the request highlighting color from grey to blue (euiColorPrimary) since we are soon starting work on the Console redesign project, in which the new request highlighting color is blue (see screenshot from Figma project below). With the blue color, the contrast seems to be much higher.

Screenshot from Figma project

[ElenaStoeva]

/ci

[ElenaStoeva]

/ci

[elasticmachine]

Pinging @elastic/kibana-management (Team:Kibana Management)

[sabarasaba]

Thanks for patching this up, huge QOL improvement its been bothering me for a while!

[kibana-ci]

💚 Build Succeeded

• Buildkite Build
• Commit: a17e33f

Metrics [docs]

✅ unchanged

History

• 💔 Build #224694 failed 3b96ab8
• 💔 Build #224580 failed 5d2dda1
• 💚 Build #223889 succeeded f79cb82
• 💔 Build #223722 failed fadc099
• 💔 Build #223461 failed 7a2d36c

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @ElenaStoeva