Geany is good Python source code editor in as it fulfills my required features (i) code-folding and (ii) dark colour theme.
By default, Geany only shipped with white color background. However, different color schemes can be downloaded from here. Currently I am using the geany-dark-scheme.
*In Ubuntu, one of parameters of the config file “filetypes.python” should be: stringeol=0xE23636;0xe0c0e0;false;false.