Dear milfan!

I will try later to change the
table.normaltab ul.thread: here we are (font-size:1em !important)
to see how it works.

I wouldn't do that (it will change the font size of the entire list within the table). The problem with the font-size in the subject line's heading

<h1 class="postingheadline">foo</h1>

is a reference to a class which doesn't exist in style.css. Either you add something like

.postingheadline { font-size:1.1em; }

(you can experiment with the value of font-size), or you wait for the next version.
If you change the font-size, always use the unit "em", not "px" or the like...

btw when you say "look at the XHTML source code", you mean to view the source code of the main page index.php,

No, I meant open the page you want (the main page, a thread, a single post,...) in the browser and look at the XHTML which is produced by the scripts. I don't know which browser you are using, probably it's something like [View -> Source] in your local language. That's what you are interested in.
Looking at the scripts is very confusing...

I was even puzzeling with that :)

Sure.

Also: if the admin's Forum Setting area can change those font sizes, it would be great too. Many people are not so familiar with CSS and scripts (me either).

This may go to Alex's wish-list.
(Arghl: there's a two-letter smiley rendered in this post!)