style(dark): Adjust chat bubble background color (langgenius#20990) (#21264)

web/themes/dark.css

   --color-background-gradient-bg-fill-chat-bg-2: #1d1d20;
   --color-background-gradient-bg-fill-chat-bubble-bg-1: #c8ceda14;
   --color-background-gradient-bg-fill-chat-bubble-bg-2: #c8ceda05;
-  --color-background-gradient-bg-fill-chat-bubble-bg-3: #a5bddb;
+  --color-background-gradient-bg-fill-chat-bubble-bg-3: #27314d;
   --color-background-gradient-bg-fill-debug-bg-1: #c8ceda14;
   --color-background-gradient-bg-fill-debug-bg-2: #18181b0a;