fix(md): item border has improved contrast in dark mode (#29398)

Issue number: resolves #29386

---------

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## What is the current behavior?
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The item border does not have sufficient contrast in dark mode making
the border almost invisible.

## What is the new behavior?
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this PR. -->

- Removed the `--ion-border-color` in the MD dark palette. Instead,
we're using the [theme
default](https://github.com/ionic-team/ionic-framework/blob/dc1172a8411f71227b713a6d7addca91537ea438/core/src/themes/ionic.theme.default.md.scss#L31).
## Does this introduce a breaking change?

- [ ] Yes
- [x] No

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## Other information

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screenshots of how the component looks before and after the change. -->


The earliest reference to the offending line Brandy and I could find was
[here](#18713 (comment)).

Author: liamdebeasi

core/src/components/item/test/a11y/item.e2e.ts

@@ -2,6 +2,35 @@ import AxeBuilder from '@axe-core/playwright';
 import { expect } from '@playwright/test';
 import { configs, test } from '@utils/test/playwright';
 
+configs({ directions: ['ltr'], palettes: ['dark'] }).forEach(({ config, screenshot, title }) => {
+  test.describe(title('item: dark palette'), () => {
+    /**
+     * This test was originally created to ensure the item border has sufficient
+     * contrast. We don't use an Axe test here because Axe not warn about color
+     * contrast on the item borders.
+     */
+    test('borders should have sufficient contrast', async ({ page }) => {
+      test.info().annotations.push({
+        type: 'issue',
+        description: 'https://github.com/ionic-team/ionic-framework/issues/29386',
+      });
+      await page.setContent(
+        `
+        <ion-list>
+          <ion-item>Item</ion-item>
+          <ion-item>Item</ion-item>
+        </ion-list>
+      `,
+        config
+      );
+
+      const list = page.locator('ion-list');
+
+      await expect(list).toHaveScreenshot(screenshot(`item-dark`));
+    });
+  });
+});
+
 configs({ directions: ['ltr'] }).forEach(({ config, screenshot, title }) => {
   test.describe(title('item: axe'), () => {
     test('should not have accessibility violations', async ({ page }) => {

core/src/components/item/test/a11y/item.e2e.ts-snapshots/item-dark-ios-ltr-dark-Mobile-Chrome-linux.png

@@ -142,7 +142,6 @@ $colors:  (
     --ion-background-color-rgb: 18, 18, 18;
     --ion-text-color: #ffffff;
     --ion-text-color-rgb: 255, 255, 255;
-    --ion-border-color: #222222;
     --ion-background-color-step-50: #1e1e1e;
     --ion-background-color-step-100: #2a2a2a;
     --ion-background-color-step-150: #363636;