jp: dark and light theme:

- `background` setting can now be used in sat.conf to indicate if the terminal use a
  `dark` or a `light` colour
- if `background` is not set or set to `auto`, the background colour is autodetected
- Jp.get_config (or CommandBase.get_config) method can now be used to retrieve a setting

doc/configuration.rst

@@ -103,6 +103,9 @@ the comments to get the meaning of each option.
     hosts_dict = {
         "example.org": {"host": "127.0.0.1"}
         }
+    ; background luminance, used by terminal frontends
+    ; can be "auto" (default) to autodetect, "dark" or "light"
+    background = auto
 
     [plugin account]
     ; where a new account must be created