style: update chart line and grid styles and colors

[nickofthyme]

Summary

Updates line and color styles.

Changes:

• Remove dashed lines for horizontal gridlines
• Set LIGHT_THEME grid lines color to #E9EDF3

No Changes required (as is currently):

• Set DARK_THEME grid lines color to #343741 (i.e. EUI.colors.lightShade)
• Bucket dashed line on hover color (i.e. crosshair.line) set to EUI.colors.darkShade

Issues

closes #2459

Checklist

• The proper chart type label has been added (e.g. :xy, :partition)
• The proper feature labels have been added (e.g. :interactions, :axis)
• All related issues have been linked (i.e. closes #123, fixes #123)
• Unit tests have been added or updated to match the most common scenarios
• The proper documentation and/or storybook story has been added or updated
• Visual changes have been tested with light and dark themes

[nickofthyme]

@gvnmagni Can you take a look at the requested changes? Here is a link to the storybook with the gridline options.

[nickofthyme]

buildkite update screenshots

[nickofthyme]

Large VRT diff (900+) due to changes in gridline dash and color styles.

[nickofthyme]

buildkite update screenshots