restructured imports from modules, created imports in __init__.py

customtkinter/windows/widgets/theme/__init__.py

@@ -0,0 +1,9 @@
+from .theme_manager import ThemeManager
+
+# load default blue theme
+try:
+    ThemeManager.load_theme("blue")
+except FileNotFoundError as err:
+    raise FileNotFoundError(f"{err}\n\nThe .json theme file for CustomTkinter could not be found.\n" +
+                            f"If packaging with pyinstaller was used, have a look at the wiki:\n" +
+                            f"https://github.com/TomSchimansky/CustomTkinter/wiki/Packaging#windows-pyinstaller-auto-py-to-exe")