Migrate and cleanup legacy scss (#69369)

src/core/public/index.scss

@@ -2,6 +2,5 @@
 @import './chrome/index';
 @import './overlays/index';
 @import './rendering/index';
+@import './styles/index';
 
-// Global styles need to be migrated
-@import '../../legacy/ui/public/styles/_legacy/_index';

src/legacy/ui/public/styles/_legacy/components/_ace_overrides.scss → src/core/public/styles/_ace_overrides.scss

@@ -1,6 +1,3 @@
-@import '@elastic/eui/src/components/call_out/variables';
-@import '@elastic/eui/src/components/call_out/mixins';
-
 // SASSTODO: Replace with an EUI editor
 // Intentionally not using the EuiCodeBlock colors here because they actually change
 // hue from light to dark theme. So some colors would change while others wouldn't.
@@ -181,7 +178,7 @@
     }
 
     &.ace_multiselect .ace_selection.ace_start {
-      box-shadow: 0 0 3px 0px $euiColorEmptyShade;
+      box-shadow: 0 0 3px 0 $euiColorEmptyShade;
     }
 
     .ace_marker-layer .ace_step {

src/core/public/styles/_base.scss

@@ -0,0 +1,58 @@
+@import '@elastic/eui/src/components/collapsible_nav/variables';
+// Application Layout
+
+// chrome-context
+// TODO #64541
+// Delete this block
+.chrHeaderWrapper:not(.headerWrapper) .content {
+  display: flex;
+  flex-flow: row nowrap;
+  width: 100%;
+  height: 100%;
+  overflow: hidden;
+}
+
+.application,
+.app-container {
+  > * {
+    position: relative;
+  }
+}
+
+.application {
+  position: relative;
+  z-index: 0;
+  display: flex;
+  flex-grow: 1;
+  flex-shrink: 0;
+  flex-basis: auto;
+  flex-direction: column;
+
+  > * {
+    flex-shrink: 0;
+  }
+}
+
+// We apply brute force focus states to anything not coming from Eui
+// which has focus states designed at the component level.
+// You can also use "kbn-resetFocusState" to not apply the default focus
+// state. This is useful when you've already hand crafted your own
+// focus states in Kibana.
+:focus {
+  &:not([class^='eui']):not(.kbn-resetFocusState) {
+    @include euiFocusRing;
+  }
+}
+
+// A necessary hack so that the above focus policy doesn't pollute some EUI
+// entrenched inputs.
+.euiComboBox {
+  // :not() specificity needed to override the above
+  input:not([class^='eui']):focus {
+    animation: none !important;
+  }
+}
+
+.euiBody--collapsibleNavIsDocked .euiBottomBar {
+  margin-left: $euiCollapsibleNavWidth;
+}

src/core/public/styles/_index.scss

@@ -0,0 +1,2 @@
+@import './base';
+@import './ace_overrides';

src/legacy/ui/public/_index.scss

@@ -1,6 +1,3 @@
-// Legacy styles to come before all
-@import './styles/_legacy/index';
-
 // Prefix all styles with "kbn" to avoid conflicts.
 // Examples
 //   kbnChart