Please explain your query with some example / screenshot.
It uses CKEditor as wysiwyg so you can also try searching on google.

The wysiwyg-editor from phpgrid adds automaticly some code like:

<span style="font-size: 13px; line-height: 20.8px;">

Where may I change this, that this code is not automaticly added?

Refer this solution: http://stackoverflow.com/a/18504875/385377

You can see ckeditor files in lib/js/ckeditor/…