fix: import DARK_CARD + use hasattr for vitis_path

- widgets.py: add DARK_CARD to gui.styles imports
- zynq_checker.py: wrap config.vitis_path with hasattr
- bitstream_programmer.py: wrap config.vitis_path with hasattr

src/gui/widgets.py

@@ -32,8 +32,8 @@ from gui.styles import (
     ACCENT_ERROR,
     ACCENT_DISABLED,
     CONNECTOR_COLOR,
-    DARK_CARD,
     DARK_FG,
+    DARK_CARD,
     CIRCLE_PENDING,
     CIRCLE_RUNNING,
     CIRCLE_COMPLETE,