By default in the astroid framework, the majority of the HTML rendered can be edited via the /frontend/ folder in your template. However, updating astroid to a newer version would overwrite your modifications with that or core astroid files.
Starting Astroid 1.1.3, You can override the frontend folder as well.
If you’d like to override the header layout file header.php
ROOT/templates/astroid_template_zero/frontend/header.php
Copy the file header.php to
ROOT/templates/astroid_template_zero/html/frontend/header.php
and that should do it, the overrides are applicable to all files under the frontend folder.