Describe the bug
Heatmap doesn't adapt to the currently running Kibana theme

To Reproduce

• Enable dark mode in Kibana
• Visit ML anomaly viewer

Expected: Heatmap adapts to theme
Actual: Heatmap does not adapt to theme

Note: Lens works around this by explicitly setting background color

Version (please complete the following information):

• OS: OSX
• Browser: chromium edge 92
• Elastic Charts: version @ Kibana 7.15 BC1

Kibana Cross Issues
elastic/kibana#97636

Checklist

Delete any items that are not applicable to this issue.

• every related Kibana issue is listed under Kibana Cross Issues list
• kibana cross issue tag is associated to the issue if any kibana cross issue is present