Flamegraph: Fix css issues when embedded outside of Grafana

[aocenas]

What is this feature?

• Fix z-index being high for search input
• Fix tooltip table max-width
• Set top margin for tooltip item text
• Change the color of muted collapsed nodes so they don't stand up too much

• Add some gap between graph and table and also add subtle background to the table so it does not blend in with the graph.

• Use fallback color for diff mode where one side of the diff does not have data.

See: grafana/pyroscope#2346 (comment)

Why do we need this feature?

[Add a description of the problem the feature is trying to solve.]

Who is this feature for?

[Add information on what kind of user the feature is for.]

Which issue(s) does this PR fix?:

Fixes #

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[Rperry2174]

looks much better thx!