By default, the thank you page's text and other content is greyed out to make the headings pop out a bit more. This includes for example the name of your organisation.
If you don't like this however, you can edit the page's text by clicking on Edit on the page or the text field.
Simply delete everything in the field, and add them back in (don't copy, though). This will get rid of certain styles that are set in the content box's source code (this can be viewed from Tools > Source code in the field's menus).
Remember to click Save once you're finished.