[Amsterdam] Improve modals

[johnbarrierwilson]

Summary

This PR removes the 1px border on modal panels to better match the rest of the k8/amsterdam theme. It also improves overlay color by going with a darker shade instead of lighter.

Before	After

Checklist

• Check against all themes for compatibility in both light and dark modes
• Checked in mobile
• Checked in IE11 and Firefox
  - [ ] Props have proper autodocs
  - [ ] Added documentation 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_3515/

[cchaos]

I really like the way this draws attention away from the normal content and to the overlaid component (modal, flyout, etc). However, I'm wondering if this should be an option within the component as to whether to render dark or light.

For example, there's one component usage of EuiOverlayMask that the darker background doesn't work with; EuiImage. When in full screen mode, the component displays the caption below the image in regular text color (dark gray). It's no longer visible with this darker mask.

[snide]

However, I'm wondering if this should be an option within the component as to whether to render dark or light.

I'd stick with one or the other if we can. Otherwise we'll see too much subjectivity in the decisions downstream. Either is fine. The answer might just be to adjust the caption to have a background to call it out so that it works there.

I'm indifferent to the actual coloring itself. It's very subjective which style people prefer.

[cchaos]

Yeah I waffled a bit about suggesting that. I can see arguments for both. I would hate though to have to force a background color on the caption (box within box effect). Maybe just another Amsterdam override.

[johnbarrierwilson]

How's this? Using $euiColorGhost as the caption color regardless of theme.

Light Theme	Dark Theme	WCAG

[kibanamachine]

Preview documentation changes for this PR: https://eui.elastic.co/pr_3515/

[cchaos]

😎 Cool! Thanks for making that change. LGTM!