[KevinH]

And unfortunately, that invert impacts image / pngs and inverts their colour to make things look very very strange.

Instead, you can assign a Sigil clip that pastes:

  <style>
    body {
      background-color: initial !important;
      color: initial !important;
      filter: invert(1);
    }  
  </style>

into the head tag of any single xhtml page, or alternatively pastes a simpler clip added to any css file. So you can enable this when needed all with a single click on its clip, and even remove it with a single click using a Saved Search Find and Replace.

So adding an option to do this is probably not worthwhile especially given its potentially negative impact on images and issues with linux.

Try creating it as a clip and building a saved search to easily remove them. That should be be doable.