feat: expose classnames

[Caele]

Motivation

Adds static classnames to charts:

• title
• subtitle
• footer
• action buttons

To allow for some basic re-styling of embedded charts through css:
Example:

div.njs-cell {
    background: maroon;
}

p.njs-cell-footer, h6.njs-cell-title, p.njs-cell-sub-title {
    color: white;
}

button.njs-cell-action {
    color: white;
}

This would be a workaround until a proper full UI. theme can be exposed