Change user-stylesheet default

This shows scrollbars like the one in the code block here:
https://devtalk.nvidia.com/default/topic/918822/linux/hevc-main-10-profile-decoding-using-vdpau-unsupported-on-gtx-950-361-28-/

qutebrowser/config/configdata.py

@@ -314,7 +314,8 @@ def data(readonly=False):
 
             ('user-stylesheet',
              SettingValue(typ.UserStyleSheet(none_ok=True),
-                          '::-webkit-scrollbar { width: 0px; height: 0px; }',
+                          'html > ::-webkit-scrollbar { width: 0px; '
+                          'height: 0px; }',
                           backends=[usertypes.Backend.QtWebKit]),
              "User stylesheet to use (absolute filename, filename relative to "
              "the config directory or CSS string). Will expand environment "