Add theming support

[paddymul]

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Display (is this related to visual display of a value)

Enhancement Description

Buckaroo has a default dark-mode look. This works for my uses. It would be good to add theming and styling support.

This could probably go in two phases, first - light/dark switch. Then more comprehensive theming from the python side.

Pseudo Code Implementation

Theming probably falls into another baselevel key for dfviewer_config

export type GenericViewerConfig {
  theme: "dark" | "light";
}

export interface DFViewerConfig {
  pinned_rows: PinnedRowConfig[];
  column_config: ColumnConfig[];
  extra_grid_config?: GridOptions;
  generic_config?: GenericViewerConfig
}

Later it could look like this

export type GenericViewerConfig {
  theme: "dark" | "light";
  base_css_classname?: string;
  extra_css?: string;
  extra_css_url?: string

}

Prior Art

Not so much prior art but implementation plan.

This is the type of task that someone who knows more about styling and cares more about it should implement.