Added feature to change background color and text color.

At some point I would like to combine the text color and background
color so that all the user has to do is change the background color and
it will automatically change the foreground color to a good color that
contrasts the background color.
Enabled more flexibility to the custom difficulty arguments.

User can now do “custom <int>”, “custom <int> <int>”, “custom <int>
<int> <String>”, and “custom <int> <int> <String> <int>” without giving
an error. Also added random operation choosing if the user gives
multiple operations for that particular argument.
Updated private.xml