[Serverless/Side Nav] Light mode for side nav background

[tsullivan]

Summary

This removes the dark background of the side nav for serverless projects.

Pulled from #153508

Context from @MichaelMarcialis

current plans for the next revision is to ditch the high contrast nav and header in favor of one that honors the current theme mode (light for light and dark for dark).

Screenshots

Before

After

[elasticmachine]

Pinging @elastic/appex-sharedux (Team:SharedUX)

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: dd825ff

Failed CI Steps

• FTR Configs #42

Metrics [docs]

Page load bundle

Size of the bundles that are downloaded on every page load. Target size is below 100kb

id	before	after	diff
core	362.5KB	362.3KB	-127.0B

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
enterpriseSearch	19	21	+2
securitySolution	399	402	+3
total			+5

Total ESLint disabled count

id	before	after	diff
enterpriseSearch	20	22	+2
securitySolution	479	482	+3
total			+5

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

[sebelga]

LGTM 👍