[APM] Span detail banner unreadable using dark mode theme

[MeijerM1]

Kibana version:
7.16.3

Elasticsearch version:
7.16.3

Server OS version:
n/a

Browser version:
Google Chrome 100.0.4896.127 (64-bits)

Browser OS version:
MS Windows 10 Enterprise 10.0.19044 Build 19044

Original install method (e.g. download page, yum, from source, etc.):
Kubernetes install via ECK 2.1

Describe the bug:
Some information is displayed in a banner when viewing span details. The lack of contrast between the banner background and the text makes the text unreadable.

Steps to reproduce:

1. Generate some data to view in APM that is displayed in the banner (i.e. MySQL trace data using Open Telemetry for dotnet)
2. View the span details in APM

Expected behavior:
The text is readable.

Screenshots (if relevant):

Errors in browser console (if relevant):
n/a

Provide logs and/or server output (if relevant):
n/a

Any additional context:
This bug is related to the dark mode theme. This bug does not occur when using the default 'light' theme.

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[dannycroft]

@boriskirov is this something you want to take a look at?

[boriskirov]

Fixed in this pull request.