I want to change the appearance of the Title of my forms in XOOPS a little. Where can I add class="head3"?
I can't find out where/how to do this. Is there a template? Which CSS do I have to change?
Please help.

If you want to do this via css you need to change the th{} element in your theme(s) stylesheet. This of course will affect all table headers in your site, not just the ones used in forms.

If you want to alter just the XOOPS forms you will have to edit the render() function in /class/xoopsform/themeform.php - this is where the tag is implemented - around line 82.

Of course not all modules draw their forms using the XOOPS form classes so you may have to edit some modules by hand via their templates and/or php files.

Venezia