The default Auto mode gives the widget no background of its own — your page shows straight through it. Dark mode, light mode, cream, or any custom color you set. Text contrast is picked to match. Nothing to configure.
Auto already follows the reader’s system setting. If your theme flips light/dark with its own button, add this one line anywhere on the page and the widgets will follow that button too.
Optional. Nothing to configure — it finds the widgets on the page by itself.