Add predefined themes

[Bedis Nbiba]

Currently the user can tailor the theme to his liking by changing each item in theme.toml, but this doesn't allow predefined themes.

I propose translating to this themes structure:

--config--themes-------------|--default
         |-theme.toml        |---theme1
                             |---theme2

where theme.toml contains the theme name which is located under themes, and each theme have the items configuration.

It's up to discussion what should happen when the user use :color command, maybe it should create immediately a custom theme called customxxx1.toml with the change and the next changes during the repl session.