Fix light_theme changes default node background

This is an issue where the background of nodes which have no background set were being lightened by this switch, when they should be skipped.

Went unnoticed because the only theme using this was the built-in light theme, which used white for node backgrounds anyway.

src/scripts/app.ts

@@ -658,7 +658,9 @@ export class ComfyApp {
         }
       }
 
-      node.bgcolor = adjustColor(bgColor, adjustments)
+      if (old_bgcolor) {
+        node.bgcolor = adjustColor(old_bgcolor, adjustments)
+      }
 
       // @ts-expect-error fixme ts strict error
       const res = origDrawNode.apply(this, arguments)