Set default pane background colour to white

After MR:

There may be some css fixes that the iframe needs to perform to use more neutral choices. We're kind of shoehorning the preview iframe into a space that was built to provide a dark background.

I suppose we could consider this like a reset/normalise css to at least get the preview pane closer to the default state of a browser? Could force this to be the themer's problem of course just thought it would be a simple fix to make this work better out for the box without developer intervention.

Scott:

Thanks for the quick reply.

I agree we could do more to make this pane work for us. I think we need to explore a number of different avenues to make this happen.

At this point, I think I should restate our high-level goal of getting this work into core and as a result, we don't want to "break the box" too much. So we can find a solution that works with core's existing Off Canvas Dialog (that we're using for this) that would be best.

And if we do need to break out of that solution, that we have ample justification for doing so.

Makes sense, I suggest I open this as an eventual Core issue then after module merge perhaps. By that time I assume setup might be different and since this is a trivial change makes sense to close.