Customizing custom themes

Hi,

I’m trying to customize a theme for our newsletter mailings. I’m following the instructions here, but they are a little thin:
https://www.thenewsletterplugin.com/documentation/newsletter-themes

Does anyone know of other documentation or tutorials on this subject?

I followed the instructions to duplicate the default theme to a custom theme, and that is complete.

But I can’t figure out how to find or set the various $theme_options such as:
$theme_options['main_header_title'] $theme_options['theme_header_logo']['url'] $theme_options['main_header_sub'] and similar.

I also don’t understand:

$theme_options – is an associative array with theme options. Each key on this file must be prefixed by “theme_”.

How do I find the options or add to the array? same with $controls

Also puzzled by what this means:

Within theme-options.php, an instance of this class is available in the variable $controls and should be used to build the theme configuration panel.

I’m probably missing something obvious… can someone point me in the right direction, or offer tips and suggestions?