Add background to data grid content. Add more params to euiScrollBar

[snide]

Summary

• Adds a background to EuiDataGrid content. This is helpful for when it is in virutalized mode.
• Added two params to mixin euiScrollBar to allow for customizing coloring.
• Removed the SCROLLBAR_HEIGHT as a factor in sizing height. As far as I can tell in FF/Chrome, this was adding unnecessary height.

After

Before

I will consult with @cchaos later today for some styling feedback.

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in Chrome, Safari, Edge, and Firefox
• [ ] Props have proper autodocs and playground toggles
• Added documentation
• Checked Code Sandbox works for the any docs examples
• Added or updated jest tests
• Checked for breaking changes and labeled appropriately
• Checked for accessibility including keyboard-only and screenreader modes
• A changelog entry exists and is marked appropriately

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4509/

[snide]

@chandlerprall This is a pretty simple one, but can you please verify that removing the scrollbar height is ok? I couldn't find any usage where it needed to be part of the calculation. It looked like it was extraneous.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4509/

[chandlerprall]

Woohoo! Love that scrollbar height going away, confirmed it isn't needed anymore.

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4509/

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4509/