[Lens] Outer labels invisible on nested treemap

[flash1293]

Kibana version: 7.11

Describe the bug: When configuring a treemap with two group by dimensions, the labels of the outer dimension are not shown in the chart.

Steps to reproduce:

1. Go to Lens
2. Switch to treemap
3. Pull in two string fields
4. Only inner labels are visible, with empty space reserved for outer labels

Expected behavior:
Should show outer labels as well

Screenshots (if relevant):
What's the meaning of the colors?

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[wylieconlon]

Looks like this was caused by the recent PR here: #83061

[dej611]

It seems like a bug on the charts library side: I can reproduce it without the #83061 PR applied or on the elastic-charts side as well. :(

[flash1293]

@dej611 Could you open a cross-issue with a reproducible example? I checked the storybook here and it seems like it's still possible to style treemaps correctly, so I guess this is something about the default theme that changed recently.

[dej611]

The elastic-charts library sets the text color to white when the background color is transparent, which is not "wrong". Perhaps "black" could be a better default value here?

I'm currently trying to articulate what's the desiderata from the Lens prospective: in all their storybook examples the textColor property is set for each layer, with the color dynamically computed only if there isn't enough contrast (textInvertible: true), which is not possible in Lens (I suspect).

More in general I think the theme background should be passed over to the elastic-charts library in Lens to make it compute the best text color to comply.