[Security Solution] Expandable flyout - update preview shadow (#211098)

## Summary

Improved UI on preview background

Before


![image](https://github.com/user-attachments/assets/5ae595a3-ebdc-4a18-a58d-6e65eb61574a)


After

![image](https://github.com/user-attachments/assets/e8f9aafb-6bcc-42e3-ab92-0c394be4d695)



### Checklist

- [x] The PR description includes the appropriate Release Notes section,
and the correct `release_note:*` label is applied per the
[guidelines](https://www.elastic.co/guide/en/kibana/master/contributing.html#kibana-release-notes-process)

x-pack/solutions/security/packages/expandable-flyout/src/components/preview_section.tsx

@@ -13,6 +13,7 @@ import {
   EuiSplitPanel,
   EuiText,
   useEuiTheme,
+  transparentize,
 } from '@elastic/eui';
 import React, { memo, useMemo } from 'react';
 import { css } from '@emotion/react';
@@ -134,21 +135,17 @@ export const PreviewSection: React.FC<PreviewSectionProps> = memo(
       <div
         css={css`
           position: absolute;
-          top: 8px;
-          bottom: 8px;
-          right: 4px;
+          top: 0;
+          bottom: 0;
+          right: 0;
           width: ${width};
           z-index: 1000;
+          padding: ${euiTheme.size.m} ${euiTheme.size.s} 0px ${euiTheme.size.s};
+          // TODO EUI: add color with transparency
+          background: ${transparentize(euiTheme.colors.shadow, 0.1)};
         `}
       >
-        <EuiSplitPanel.Outer
-          css={css`
-            margin: ${euiTheme.size.xs};
-            box-shadow: 0 0 ${euiTheme.size.base} 0 ${euiTheme.colors.lightShade};
-          `}
-          data-test-subj={PREVIEW_SECTION_TEST_ID}
-          className="eui-fullHeight"
-        >
+        <EuiSplitPanel.Outer data-test-subj={PREVIEW_SECTION_TEST_ID} className="eui-fullHeight">
           {isPreviewBanner(banner) && (
             <EuiSplitPanel.Inner
               grow={false}

x-pack/solutions/security/plugins/security_solution/public/timelines/components/modal/index.styles.tsx

@@ -27,6 +27,7 @@ export const usePaneStyles = () => {
     left: 0;
     right: 0;
     bottom: 0;
+    // TODO EUI: add color with transparency
     background: ${transparentize(euiTheme.colors.ink, 0.5)};
     z-index: ${(euiTheme.levels.flyout as number) +
     1}; // this z-index needs to be between the eventFlyout (set at 1000) and the timelineFlyout (set at 1002)