Dark Themes

When a notebook or application is rendered using a dark theme, DataTable requires that a dark class be added to the HTML document. This can be done with the following Javascript snippet:

document.documentElement.classList.add('dark');

When ITables is used in a notebook, this is handled by init_notebook_mode which displays the init_datatables.html snippet.

Please open a PR if you see how to further improve the support of light vs dark themes, and e.g. set the dark class dynamically when the theme is changed.