docs: add dark mode support for Algolia DocSearch (#5109)

## Summary
- Adds dark mode CSS variables for Algolia DocSearch modal
- Uses Material Design variables for consistent theming with existing
dark mode
- Implements subtle shadow and background colors appropriate for dark
theme
- Uses consistent dark mode selector pattern matching existing codebase

## Test plan
- [x] Test search modal in light mode (unchanged)  
- [x] Test search modal in dark mode (now properly styled)
- [x] Verify colors match overall Material Design dark theme

## Before

<img width="595" height="222" alt="image"
src="https://github.com/user-attachments/assets/16dd3d15-53fe-43fc-9d00-80150f083145"
/>

<img width="1049" height="345" alt="image"
src="https://github.com/user-attachments/assets/2f1a91cf-55ea-4df7-ad28-ddd9b114a48d"
/>

## After

<img width="584" height="228" alt="image"
src="https://github.com/user-attachments/assets/2be1afe8-2ccf-47c2-a13e-9cc525debd35"
/>

<img width="1032" height="341" alt="image"
src="https://github.com/user-attachments/assets/bcb98d6c-073e-4ed2-b1b1-7f86f03ec5a7"
/>

Author: ykdojo

docs/css/docsearch.css

@@ -67,3 +67,21 @@
 .DocSearch-Help {
     font-size: 1em !important
 }
+
+/* DARK MODE OVERRIDES */
+[data-md-color-scheme="slate"][data-md-color-primary="black"] {
+    --docsearch-text-color: var(--md-default-fg-color);
+    --docsearch-container-background: rgba(0, 0, 0, 0.6);
+    --docsearch-modal-background: var(--md-default-bg-color);
+    --docsearch-modal-shadow: 0 8px 32px rgba(0, 0, 0, 0.8);
+    --docsearch-searchbox-background: var(--md-code-bg-color);
+    --docsearch-searchbox-focus-background: var(--md-code-bg-color);
+    --docsearch-hit-color: var(--md-default-fg-color);
+    --docsearch-hit-active-color: var(--md-default-bg-color);
+    --docsearch-hit-background: transparent;
+    --docsearch-hit-shadow: none;
+    --docsearch-footer-background: var(--md-default-bg-color);
+    --docsearch-footer-shadow: inset 0 1px 0 0 rgba(73, 76, 106, 0.2);
+    --docsearch-spacing: 12px;
+    --docsearch-muted-color: var(--md-default-fg-color--light);
+}