migrate style overrides from scss to leverage emotion (elastic#204463)

## Summary

This PR migrates style overrides within core from SCSS to leverage
emotion. Why is this necessary? Kibana builds it's stylesheets we get 4
stylesheets for all the supported themes, in which in turn impacts the
files built. Switching to leveraging emotion makes it such that we don't
need to build 4 files anymore for this particular cases in turn reducing
the page load bundle size.

This PR results in ~12% reduction in the page load bundle, that would
have come into effect from [the
PR](elastic#203840) that introduces the
new Borealis theme as a default. The results can be seen in [this
build](https://buildkite.com/elastic/kibana-pull-request/builds/260881)

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Co-authored-by: Elastic Machine <elasticmachine@users.noreply.github.com>

packages/core/application/core-application-common/index.ts

@@ -10,3 +10,4 @@
 export type { AppCategory } from './src/app_category';
 export { APP_WRAPPER_CLASS } from './src/app_wrapper_class';
 export { DEFAULT_APP_CATEGORIES } from './src/default_app_categories';
+export { GlobalAppStyle } from './src/global_app_style';

packages/core/application/core-application-common/src/global_app_style.tsx

@@ -0,0 +1,164 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the "Elastic License
+ * 2.0", the "GNU Affero General Public License v3.0 only", and the "Server Side
+ * Public License v 1"; you may not use this file except in compliance with, at
+ * your election, the "Elastic License 2.0", the "GNU Affero General Public
+ * License v3.0 only", or the "Server Side Public License, v 1".
+ */
+
+import React from 'react';
+import { css, Global } from '@emotion/react';
+import { useEuiTheme, type UseEuiTheme } from '@elastic/eui';
+
+export const renderingOverrides = (euiTheme: UseEuiTheme['euiTheme']) => css`
+  #kibana-body {
+    // DO NOT ADD ANY OVERFLOW BEHAVIORS HERE
+    // It will break the sticky navigation
+    min-height: 100%;
+    display: flex;
+    flex-direction: column;
+  }
+
+  // Affixes a div to restrict the position of charts tooltip to the visible viewport minus the header
+  #app-fixed-viewport {
+    pointer-events: none;
+    visibility: hidden;
+    position: fixed;
+    top: var(--kbnAppHeadersOffset, var(--euiFixedHeadersOffset, 0));
+    right: 0;
+    bottom: 0;
+    left: 0;
+  }
+
+  .kbnAppWrapper {
+    // DO NOT ADD ANY OTHER STYLES TO THIS SELECTOR
+    // This a very nested dependency happening in "all" apps
+    display: flex;
+    flex-flow: column nowrap;
+    flex-grow: 1;
+    z-index: 0; // This effectively puts every high z-index inside the scope of this wrapper to it doesn't interfere with the header and/or overlay mask
+    position: relative; // This is temporary for apps that relied on this being present on \`.application\`
+  }
+
+  .kbnBody {
+    padding-top: var(--euiFixedHeadersOffset, 0);
+  }
+
+  // Conditionally override :root CSS fixed header variable. Updating \`--euiFixedHeadersOffset\`
+  //on the body will cause all child EUI components to automatically update their offsets
+  .kbnBody--hasHeaderBanner {
+    --euiFixedHeadersOffset: var(--kbnHeaderOffsetWithBanner);
+
+    // Offset fixed EuiHeaders by the top banner
+    .euiHeader[data-fixed-header] {
+      margin-top: var(--kbnHeaderBannerHeight);
+    }
+
+    // Prevent banners from covering full screen data grids
+    .euiDataGrid--fullScreen {
+      height: calc(100vh - var(--kbnHeaderBannerHeight));
+      top: var(--kbnHeaderBannerHeight);
+    }
+  }
+
+  // Set a body CSS variable for the app container to use - calculates the total
+  // height of all fixed headers + the sticky action menu toolbar
+  .kbnBody--hasProjectActionMenu {
+    --kbnAppHeadersOffset: calc(
+      var(--kbnHeaderOffset) + var(--kbnProjectHeaderAppActionMenuHeight)
+    );
+
+    &.kbnBody--hasHeaderBanner {
+      --kbnAppHeadersOffset: calc(
+        var(--kbnHeaderOffsetWithBanner) + var(--kbnProjectHeaderAppActionMenuHeight)
+      );
+    }
+  }
+
+  .kbnBody--chromeHidden {
+    // stylelint-disable-next-line length-zero-no-unit
+    --euiFixedHeadersOffset: 0px;
+
+    &.kbnBody--hasHeaderBanner {
+      --euiFixedHeadersOffset: var(--kbnHeaderBannerHeight);
+    }
+
+    &.kbnBody--hasProjectActionMenu {
+      --kbnAppHeadersOffset: var(--euiFixedHeadersOffset, 0);
+    }
+  }
+`;
+
+export const bannerStyles = (euiTheme: UseEuiTheme['euiTheme']) => css`
+  .header__topBanner {
+    position: fixed;
+    top: 0;
+    left: 0;
+    height: var(--kbnHeaderBannerHeight);
+    width: 100%;
+    z-index: ${euiTheme.levels.header};
+  }
+
+  .header__topBannerContainer {
+    height: 100%;
+    width: 100%;
+  }
+`;
+
+export const chromeStyles = (euiTheme: UseEuiTheme['euiTheme']) => css`
+  .euiDataGrid__restrictBody {
+    .headerGlobalNav,
+    .kbnQueryBar {
+      display: none;
+    }
+  }
+
+  .euiDataGrid__restrictBody.euiBody--headerIsFixed {
+    .euiFlyout {
+      top: 0;
+      height: 100%;
+    }
+  }
+
+  .chrHeaderHelpMenu__version {
+    text-transform: none;
+  }
+
+  .chrHeaderBadge__wrapper {
+    align-self: center;
+    margin-right: ${euiTheme.size.base};
+  }
+
+  .header__toggleNavButtonSection {
+    .euiBody--collapsibleNavIsDocked & {
+      display: none;
+    }
+  }
+
+  .header__breadcrumbsWithExtensionContainer {
+    overflow: hidden; // enables text-ellipsis in the last breadcrumb
+    .euiHeaderBreadcrumbs {
+      // stop breadcrumbs from growing.
+      // this makes the extension appear right next to the last breadcrumb
+      flex-grow: 0;
+      margin-right: 0;
+
+      overflow: hidden; // enables text-ellipsis in the last breadcrumb
+    }
+  }
+  .header__breadcrumbsAppendExtension {
+    flex-grow: 1;
+  }
+`;
+
+export const GlobalAppStyle = () => {
+  const { euiTheme } = useEuiTheme();
+  return (
+    <Global
+      styles={css`
+        ${bannerStyles(euiTheme)} ${chromeStyles(euiTheme)} ${renderingOverrides(euiTheme)}
+      `}
+    />
+  );
+};

packages/core/rendering/core-rendering-browser-internal/src/rendering_service.tsx

@@ -20,6 +20,7 @@ import type { ThemeServiceStart } from '@kbn/core-theme-browser';
 import type { UserProfileService } from '@kbn/core-user-profile-browser';
 import { KibanaRootContextProvider } from '@kbn/react-kibana-context-root';
 import { APP_FIXED_VIEWPORT_ID } from '@kbn/core-rendering-browser';
+import { GlobalAppStyle } from '@kbn/core-application-common';
 import { AppWrapper } from './app_containers';
 
 interface StartServices {
@@ -62,6 +63,9 @@ export class RenderingService {
     ReactDOM.render(
       <KibanaRootContextProvider {...startServices} globalStyles={true}>
         <>
+          {/* Global Styles that apply across the entire app */}
+          <GlobalAppStyle />
+
           {/* Fixed headers */}
           {chromeHeader}
 

src/core/public/styles/_index.scss

@@ -1,3 +1 @@
 @import './base';
-@import './chrome/index';
-@import './rendering/index';