Change category cards to light grey background for better contrast

- Replace white background with light grey (#f8f9fa)
- Provides better visual separation from white page background
- Maintains clean, modern flat design

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: tomvannuenen

static/stylesheets/custom-home.css

@@ -7,7 +7,7 @@
 
 /* Modern flat design for category cards */
 .category-card {
-  background: #ffffff;
+  background: #f8f9fa;
   transition: transform 0.3s ease, box-shadow 0.3s ease;
 }