How to add a tagline or sub-heading to the blog heading?

You'll want to fork an editable copy of your template if you haven't already and then add a new line after line 6 of header.html containing something like this:

<br><span style="color:red;font-size:14px">by XYZ</span>

Does that get you started? Please let me know if you have any questions about this or anything else

thank you! it did work but is appearing after the avatar (which I've selected to show) rather than right after the blog title. So there's quite a bit of space between the blog title and the tagline, and also the tagline is indented at the avatar rather than at the blog title text.

Please let me know if it's possible to show it just below the title and indented with the title text. thank you!! :)

For now, I removed the br tag and kept it all on one line but I don't like it very much...

I'd wrap the {{title}} tag and new <br> and <span> tags in a parent node, e.g. another span tag to prevent the spacing issues you describe:

<a id="logo" href="/">
      <img class="avatar {{#roundAvatar}}rounded{{/roundAvatar}}" src="{{avatar}}" alt="{{title}}"/>
      <span>
          {{title}}<br>
          <span style="font-size:14px">by XYZ</span>
     </span>
    </a>

I tried this but it's still the same.. :( For now, I've dropped the idea of adding my name there - looks cleaner without it.. so it's cool. thanks for your help as always!