Update app.py

app.py

@@ -19,10 +19,10 @@ css = """
     border-radius: 6px !important;
     border-color: #21293b !important;
 }
-.user {
+.user.dark {
     background: #1e293b !important;
 }
-.assistant, .pending {
+.assistant.dark, .pending.dark {
     background: #0f172a !important;
 }
 """