Is your feature request related to a problem? Please describe.
Currently I can style only the whole chart via theme prop in <Settings /> component as described in the storybook.
But the TSVB currently has a possibility to style each series separately:

Describe the solution you'd like
I think each series should have its own style option, which should have fill, line width and point size fields inside:
e.x. <LineSeries theme={ { fill: 0.5, lineWidth: 2, pointSize: 3} }

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
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Kibana Cross Issues
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Checklist

• this request is checked against already exist requests
• 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