list is a protected keyword in more recent versions of PHP, so you cannot use it as a method name.

Use list_ instead, and then add the @subcommand list annotation to the docblock, similar to how it is done here: https://github.com/wp-cli/extension-command/blob/master/src/Plugin_Command.php#L1006-L1008

Setting the default --format should be unnecessary, as the default is already provided in https://github.com/wp-cli/extension-command/pull/100/files#diff-03f3f9875c58703b8a8855ca3e5ca77eR138 and will be enforced by the system.

Should be Gets a list of theme mods. to match similar commands.

This one should be removed, as the filtering is overridden by the --all flag.

The tests should be written with a list of theme mods that actually contain values, otherwise the tests could just succeed by random chance.

You can use something like this to pre-populate the list:

Background:
  Given a WP install
  And I run `wp theme mod set key_a value_a`
  And I run `wp theme mod set key_b value_b`

@schlessera Done. can you please check again? 😃

Code Style: Always end files with a new line to avoid text processing issues on exotic platforms.

Done.....

Done

Now that I think about this some more, I'd prefer to have it just always forcefully set $assoc_args['all'] to 1 here. If you check for existence of the key first, a user could do something like this, which would produce an error:

wp theme mod list --no-all

So please just remove the if ( ! array_key_exists( ... ) { } that wraps the assignment.