It looks like the theme option is not being set. When you initialize tablesorter without any options:
$("#myTable").tablesorter();
The "default" theme is set, so make sure to include the "theme.default.css" file; but since it looks like you are loading the "blue" theme stylesheet, initialize the plugin as follows:
<script type="text/javascropt">
$(function(){
$("#myTable").tablesorter({
theme: "blue"
});
});
</script>
Now, from looking at the blue theme file name, it appears that it might be the original blue theme meant for tablesorter v2.0.5 ("/blue/style.css"). I would venture to guess that the tablesorter being used here is from my fork of tablesorter, so make sure to load the file named "theme.default.css".