Update background color to soft warm off-white

Set --ifm-background-color to #F8F9F5 for a softer, more pleasant
background that reduces eye strain while maintaining readability.

Author: claude

src/css/custom.css

@@ -13,6 +13,7 @@
   --ifm-color-primary-light: #2e7637;
   --ifm-color-primary-lighter: #307b39;
   --ifm-color-primary-lightest: #35863e;
+  --ifm-background-color: #F8F9F5;
   --ifm-code-font-size: 95%;
   --docusaurus-highlighted-code-line-bg: rgba(0, 0, 0, 0.1);
 }