Can’t figure out how to center tags on a tumblr theme? padding issue

It may be tedious, but it looks like you'll have to include the HasTags block in each of the section types.

Starting with the first you'll end up with this:

    {block:Text}
       <li class="post text">
           <!-- HTML layout for Text posts -->

        {block:HasTags}
            {block:Tags}
                <a href="{TagURL}">{Tag}</a>
            {/block:Tags}
        {/block:HasTags}

       </li>
    {/block:Text}

Where you currently have it appear just before {/block:Posts}, change it to be just before the closing </li> which appears to be what your post container styles are dressed in.

Don't forget to remove the block that appears just before the closing {/block:Posts} or you'll get the tags showing up twice.

I've got another idea.

{block:Tags}<a class="tag" href="{TagURL}">{Tag}</a><span class="tagtail"></span>{/block:Tags}

This will add a span with the "tagtail" class after every tag.

Then, add this css:

span.tagtail + a.tag:before {
   content:", ";
}

This will select every tag anchor that comes after a tagtail span (so every one but the first). This way, we avoid the need to use the first-child or last-child selectors.

UPDATE:

If you want to ensure that the commas are styled apart from the anchor, I suppose you could also do:

{block:Tags}<span class="taghead">, </span><a class="tag" href="{TagURL}">{Tag}</a>{/block:Tags}

and then style it like:

span.taghead { display:none; }
a.tag + span.taghead { display:inline; }

However, this might need tweaking because browsers differ on overriding rules.