fix(site): restore correct --sl-color-black in light mode for search box contrast (#646)

Agent-Logs-Url: https://github.com/githubnext/ado-aw/sessions/450f8317-4c5f-4a92-ac6d-c549d6577d12

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: jamesadevine <4742697+jamesadevine@users.noreply.github.com>

Author: Copilot

site/src/styles/custom.css

@@ -35,7 +35,7 @@
   --sl-color-accent: #6b46c1;
   --sl-color-accent-high: #4c2889;
   --sl-color-text-accent: #6b46c1;
-  --sl-color-black: #1f2328;
+  --sl-color-black: #ffffff;
   --sl-color-text: #1f2328;
   --sl-color-bg: #ffffff;
   --sl-color-bg-sidebar: #f6f8fa;