docs: set the background color of platform icons to transparent (#4377)

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-->

## Summary by CodeRabbit

- **Bug Fixes**
- Resolved an issue where platform icons displayed with a black
background in dark theme by ensuring the background remains transparent.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

Author: njzjz

doc/_static/css/custom.css

@@ -18,5 +18,7 @@ html[data-theme="dark"] {
   }
   img.platform-icon {
     filter: invert(1);
+    /* otherwise the backgorund is black */
+    background-color: transparent !important;
   }
 }