[EuiThemeProvider] Allow nested `modify.breakpoint` overrides

[cee-chen]

Summary

closes https://github.com/elastic/eui/issues/7829

I originally started this work hoping it would be a quick fix/workaround solution to the broken docs example, but I quickly realized that EuiThemeProvider needed to be updated to handle modify={{ breakpoint }} overrides because it was currently not doing anything with that configuration.

We've also recently seen some dialogue in Slack with Fleet about needing to set custom breakpoints for certain screens/views/flyouts as opposed to entire plugins, so this work would strongly facilitate the ability to do that (currently difficult or not possible).

[!NOTE] Not addressed in this PR: "I would still consider adding guidelines to flyout in our design system. it's important so when designers use 1200 version, they know what design principles to follow" "[...] We document how to customize our breakpoints but not when to or when not to"

QA

• Go to https://eui.elastic.co/pr_7862/#/theming/breakpoints/values#custom-values
• Open your browser devtools and responsive mode
• Set the browser width to 2501px wide (or more)
• [x] Confirm the xxl breakpoint is shown in both the example and in the table:

General checklist

• Browser QA
  • [x] Checked in mobile
  • [x] Checked in Chrome, Safari, Edge, and Firefox ~- [ ] Checked in both light and dark modes~ ~- [ ] Checked for accessibility including keyboard-only and screenreader modes~
• Docs site QA
  • [x] Updated documentation ~- [ ] Props have proper autodocs (using @default if default values are missing) and playground toggles~ ~- [ ] Checked Code Sandbox works for any docs examples~
• Code quality checklist
  • [x] Added or updated jest ~and cypress tests~ ~- [ ] Updated visual regression tests~
• Release checklist
  • [x] A changelog entry exists and is marked appropriately. ~- [ ] If applicable, added the breaking change issue label (and filled out the breaking change checklist)~
• Designer checklist - N/A

[kibanamachine]

Preview staging links for this PR:

• Docs site: https://eui.elastic.co/pr_7862/
• Storybook: https://eui.elastic.co/pr_7862/storybook
• New docs: https://eui.elastic.co/pr_7862/new-docs

[elasticmachine]

:green_heart: Build Succeeded

• Buildkite Build
• Commit: 5de569c3b313eb826c9911802c7387f2e60dd261

History

• :green_heart: Build #2268 succeeded 71cc628dbb26aee49bf1d2ca40ca8dc597cd0691