themes should be in build function.

main.py

@@ -40,17 +40,7 @@ def __init__(self, **kwargs):
 
         self.APP_NAME = "Hamster"
         self.COMPANY_NAME = "Hamster.org"
-
-        self.theme_cls.primary_palette = "Blue"
-        self.theme_cls.primary_hue = "500"
-
-        self.theme_cls.accent_palette = "Amber"
-        self.theme_cls.accent_hue = "500"
-
-        self.theme_cls.theme_style = "Light"
-
 
-
     def chat_room(self,touch,a):
         """Switch to Chatroom. but username and chatroom username 
         change according to which one you touch in chat list"""
@@ -85,7 +75,13 @@ def build(self):
         that need before start application all other method and code 
         write here.
         """
-        self.theme_cls.theme_style="Light"
+        self.theme_cls.primary_palette = "Blue"
+        self.theme_cls.primary_hue = "500"
+
+        self.theme_cls.accent_palette = "Amber"
+        self.theme_cls.accent_hue = "500"
+
+        self.theme_cls.theme_style = "Light"
 
         self.screen_manager = Root()
         self.screen_manager.add_widget(Login_Screen())