Hello,

We don't offer support for modification. You are allowed to do it at your own risk as per license. If you need you can hire developer to help you.

1. You can add custom style to `assets/custom/shop.css` for shop and `assets/custom/custom.css` for admin theme.

2. The shop assets are in `themes/default/shop/assets/` while admin area theme assets are in `themes/default/admin/assets/` The shop use gulp to compile and minify the assets. You can check `gulpfile.js`

3. You even can add your custom style tag in the header files too, Shop `themes/default/shop/view/header.php` & Admin `themes/default/admin/view/header.php`

Thank you