Thanks @dazik, no problem🙂 Welcome to Grav!
Just as a proof-of-concept, if you locate the Quark theme file at user/themes/quark/templates/partials/blog/date.html.twig and view the file you should see this:
<span class="blog-date">
<time class="dt-published" datetime="{{ page.date|date("c") }}">
<i class="fa fa-calendar"></i> {{ page.date|date(system.pages.dateformat.short) }}
</time>
</span>
To get at least get the field author of the current blog item to display change that file to include {{ page.header.author }}, so it looks like this:
<span class="blog-date">
{{ page.header.author }}
<time class="dt-published" datetime="{{ page.date|date("c") }}">
<i class="fa fa-calendar"></i> {{ page.date|date(system.pages.dateformat.short) }}
</time>
</span>
Nothing fancy of course, but this should be a good start for you.
A more elegant approach would be to create a new author.html.twig template file in that same folder, and then include that as a partial here:
https://github.com/getgrav/grav-theme-quark/blob/develop/templates/item.html.twig#L8-L17
Just as a heads-up, direct edits to theme files can be overwritten during theme updates, so it is a good idea to first create a child or inherited theme (which is best for smaller changes) - more info at https://learn.getgrav.org/15/themes/customization#theme-inheritance
Let me know if the above works for you.
Paul