[Borealis Theme] Next steps for charts and visualizations

[markov00]

Nice to have tasks after Borealis theme changes.

• Validate the EUI gradients and check if they adhere to the colorMode cc @gvnmagni. The idea for single hue gradient is that they need to be inverted when rendering them in dark mode: high contrast with the background is generally connected with high magnitude of the value we are depicting, low contrast means lower magnitude. When in darkmode the gradient needs to be inverted to keep this semantic.

• the elastic/charts library was aligned with Borealis through hardcoding the color hex in refactor(theme): reuse borealis token semantic elastic-charts#2588. When the eui-theme-borealis packge will be available and these colors can be used without a react context we can replace the hardcoded values with that package

[elasticmachine]

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