Revert grid background color

[snide]

Summary

This reverts the background color of the data grid content. Although the light gray works well for loading states, it isn't a good experience for fixed height grids with a low amount of items. We'll solve the loading issue in a better way in 7.13. I'm leaving the scrollbar mixin as is because I think it will be useful in other spots.

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

[chandlerprall]

❤️

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_4542/

[j-m]

I've finally pulled in grid virtualisation and was surprised to see this removed. I know this likely doesn't meet colour contrast accessibility requirements, but imo this is better than the potentially confusing gap on the last page of a grid

.euiDataGrid__content {
  background: #fdfdfd;
  box-shadow: 0 0 0 1px #f3f3f3 inset;
}