Added support for stylesheets. default and dark theme provided

photobooth/defaults.cfg

@@ -9,6 +9,8 @@ width = 1024
 height = 600
 # Hide cursor
 hide_cursor = False
+# Use specified style
+style = default
 
 [Camera]
 # Camera module to use (python-gphoto2, gphoto2-cffi, gphoto2-commandline, opencv)