jp: default to "dark" if background is not set and can't be detected

sat_frontends/jp/base.py

@@ -131,9 +131,9 @@ class Jp(object):
             for c in expected:
                 ch = sys.stdin.read(1)
                 if ch != c:
-                    # background id is not supported
+                    # background id is not supported, we default to "dark"
                     # TODO: log something?
-                    return
+                    return 'dark'
             red, green, blue = [int(c, 16)/65535 for c in sys.stdin.read(14).split('/')]
             # '\a' is the last character
             sys.stdin.read(1)