Hi,
I don't think you need to create a file called wysiwyg_custom.php. If you want to add custom CSS to your WYSIWYG editor, you need to create a CSS file called wysiwyg_custom.css in your lib directory:
cmsAdmin/lib/wysiwyg_custom.css
Any CSS rules you add to this will be used in your WYSIWYG editor.
Thanks!
Greg
Greg Thomas
PHP Programmer - interactivetools.com