Support changing default node/background color

[fedarko]

Update: plan as I understand it for now is having the background color changing be part of the second alpha release, and having the default node color changing be part of a later release.

• Support changing background color
• Support changing default node color

Just a small thing that would be nice in the future. For example, drawing nodes black (rather than with the default gray color currently used) increases the contrast significantly:

This could be useful for e.g. vision-impaired folks.

Might be a nice TODO at some point down the line after #137?

[ElDeveloper]

Whatever default we select, we should make both background color and tree color user-selectable.

[fedarko]

Agreed, having both selectable will be really nice. Just for reference, here's what a "dark mode" tree looks like in Empress ;) -- I imagine sticking with the black/gray on white default will be preferred by most users, but supporting this stuff (and #137) will make it really easy to style Emperor and Empress similarly.

[ElDeveloper]

Related to how we want to represent non-unique and non-represented features.

[fedarko]

One complication with this issue is that, if we change the default node color to black (which I think would be best IMO), then the root node's special black marker no longer stands out:

I think the way around this is changing the root's marker to be different in some other way (e.g. a square / star / unfilled circle / etc.), and maybe making the color of the root configurable as well.

[ElDeveloper]

A different shape should work well 👍

[fedarko]

Did a bit more looking into this; it looks like Emperor uses Spectrum for letting users select colors, so I think for the sake of simplicity it would be good to just use that for this.

(I'd thought it might be possible for us to re-use the vendor JS code between Emperor and Empress in the tandem plots -- letting us avoid duplicating the files unnecessarily -- but it doesn't look like that is currently possible, and I suspect that would probably be more trouble than it's worth.)