fix viewport color for transparent background (#617)

CHANGELOG.md

@@ -10,6 +10,7 @@ Note that since we don't clearly distinguish between a public and private interf
 - Store URL of downloaded assets to detect zip/gzip based on extension
 - Add optional `operator.key`; can be referenced in `IndexPairBonds`
 - Add overpaint/transparency/substance theme strength to representations
+- Fix viewport color for transparent background
 
 ## [v3.23.0] - 2022-10-19
 

src/mol-plugin-ui/skin/base/components/viewport.scss

@@ -4,7 +4,7 @@
     top: 0;
     right: 0;
     bottom: 0;
-    background: black;
+    background: $default-background;
 
     .msp-btn-link {
         background: rgba(0,0,0,0.2);