[Lens] Follow up tasks for Borealis theme

[nickofthyme]

Tasks to follow up after palettes merged in #201015

Tasks

Beta Give feedback

1. Allow selecting Kibana 7 (Amsterdam) palette from legacy palette picker.
2. Update gradients in eui to adapt to colorMode.
3. Update sample data images with new changes.
4. Find a way to fix the chart background issue. Currently we override the chart background color in the plugin/charts/theme service. However, this is not used everywhere. we cannot change the Theme in charts because we cannot make it theme aware. The best option may just be to find all chart instances and override the background color manually to set it as euiThemeVars.euiColorEmptyShade.

[elasticmachine]

Pinging @elastic/kibana-visualizations (Team:Visualizations)