Load statistics chart seems to be not compatible

[EstherAF]

Possibly related to #33.
Not sure if this is a bug, or it has been left in this color intentionally, but the color of the chart seems too light for the theme.

Testing with:

• jenkins-core at branch timja/dark-mode (jenkinsci/jenkins@86c0bd0)
• dark-theme at master (commit f31189f)

[daniel-beck]

The same applies to the memory usage chart on /systemInfo in recent Jenkins releases, probably because it's the same component.