fix(bec_designer): theming is applied on qt designer though main window plugin

bec_widgets/widgets/containers/main_window/bec_main_window_plugin.py

@@ -6,6 +6,7 @@ from qtpy.QtGui import QIcon
 from qtpy.QtWidgets import QWidget
 
 from bec_widgets.utils.bec_designer import designer_material_icon
+from bec_widgets.utils.colors import apply_theme
 from bec_widgets.widgets.containers.main_window.main_window import BECMainWindow
 
 DOM_XML = """
@@ -44,6 +45,8 @@ class BECMainWindowPlugin(QDesignerCustomWidgetInterface):  # pragma: no cover
         from qtpy.QtCore import Qt
         from qtpy.QtWidgets import QApplication
 
+        apply_theme("dark")
+
         import bec_widgets
 
         MODULE_PATH = os.path.dirname(bec_widgets.__file__)