Retrieve Gin css variables in another custom theme

Note that this is nothing we officially support.

You potentially can use

\Drupal::classResolver(GinSettings::class);
$settings->get('preset_accent_color');

to receive the accent color setting as a value in PHP. You might want to have a look at the gin_toolbar source code to have a look how we check for the accent color.

To receive the CSS3 vars you might want to look at the source code of gin_login gin_login_page_attachments_alter() function. You might only need the accent bits.

That's all I can help you with.

Cheers!