Your stylesheet switcher has a bit of a flaw in it, because the instructions you followed have the same flaw.
CODE
<a href="./switcher.php?set=styles-site" title="Default font size">
<a href="./switcher.php?set=styles-site_plus" title="Large font size">
Because the href starts with "./", the links to switcher.php are relative to the location of the page that the link is on.
On your main weblog page (http://www.andunie.net/), the full link to the stylesheet switcher ends up being:
CODE
http://www.andunie.net/switcher.php?set=styles-site
...which works just fine, because switcher.php is in the root directory of your domain.
On an individual archive page (such as
http://www.andunie.net/mt-archives/000274.php), the full link to the stylesheet switcher ends up being:
CODE
http://www.andunie.net/mt-archives/switcher.php?set=styles-site
...which does *not* work, because switcher.php is not in the /mt-archives directory of your weblog.
There's at least a few ways to deal with this problem. Since your weblog is at the root of your domain, the easiest solution is to remove the "." from the beginning of the href links, so they will be relative to the domain root instead of the page's URL:
CODE
<a href="/switcher.php?set=styles-site" title="Default font size">
<a href="/switcher.php?set=styles-site_plus" title="Large font size">
A more general solution would be to use the MTBlogRelativeURL tag:
CODE
<a href="<$MTBlogRelativeURL$>switcher.php?set=styles-site" title="Default font size">
<a href="<$MTBlogRelativeURL$>switcher.php?set=styles-site_plus" title="Large font size">
Side note: Your individual archive template does not have an alternate stylesheet 'link' tag like your main index template does. Not a big deal, but I thought I'd point it out while I was looking.
Hope this helps...