[BugFix] Enforce User Preference For Dark/Light Mode In Chart Output To API

[deeleeramone]

1. Why?:

   • The charting extension is relying on the output at runtime to set the user preference theme, this was excluding the API from being set by this preference.

2. What?:

   • Sets plot_bgcolor, paper_bgcolor and font_color as white/black according to the user-defined preference in user_settings.json, "chart_style".
   • This is done at the chart function level, prior to the layout_kwargs entry at output, and remains settable at runtime.

3. Impact:

   • Charts output from the API should now be consistent with the Python output.

4. Testing Done:

   • Define the preference in user_settings.json and created the different charts from endpoints.

[piiq]

At some point we can consider aggregating the styling in one place, but this is out of scope for this PR. Thanks

[deeleeramone]

At some point we can consider aggregating the styling in one place, but this is out of scope for this PR. Thanks

That'll be in scope when we deal with replacing frontend-components, which needs to happen at some point.