[Visualizations] Borealis theme alignment follow ups

[markov00]

This issue keeps tracks of all workarounda applied to KIbana and elastic-charts to make it work within the current state of the EUI Borealis theme.

• Vega tooltip in darkmode should have lighter text (@markov00) Fix vega tooltip color in dark mode #212042
• Fix Lens dimension background in darkmode as described here Borealis Visualization Team alignment #203050 (comment)
• Update Kibana Sample Data Cards images with new Borealis screenshots Update Sample Dashboards images to Borealis #212012
• TSVB test: even if the theme applied was borealis, the applied colors to the chart are from Amsterdam. The workaround was applied inhttps://github.com/[TSVB] fix borealis colors check on test #206484 and consisted of checking for test validity in both colors.
• Validate the EUI gradients and check if they adhere to the colorMode cc @gvnmagni
• elastic-charts color alignments: the colors are copied from EUI using the same semantic structure. When available from the eui-theme-borealis package we should use this dependency and use the colors from there.

[elasticmachine]

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