Default template for version 4

[jonmmease]

For version 4 we have the opportunity to introduce a new default template (i.e. theme). I would like to propose that we use the plotly theme as the default (See for some screenshots of the various themes), as the grey background is really nice for the kind of subplot faceting that plotly_express brings to the table.

As discussed in #1274 (comment), I would like to change the default sequential colorscale to plasma.

We should also take the opportunity to reevaluate the default colorway ordering. We added the last 4 colors after establishing the first 6, I don't have an exact proposal yet, but I think we should try to reorder them to move the orange (8) and yellow (7) further apart in the sequence. And maybe move green (10) further up the sequence since it has a nice contrast with everything else.