Is it possible to change the sourcecode of the content fragment headers?

You can edit styles via Theme Studio: Check the  Working with Theme Studio Developer Training and the section on Style Sheets. 

The "content-fragment-header", "content-fragment-content", "content-fragment-footer" and similar sections are integral to the rendering and control of the page via theming, and can't be changed.

Hi Steven, I was looking over your answer again.. I see that you are referring to the content-fragment-header/content/footer... I'm wondering if we are talking about the same thing here...
When I say ''content-fragment-header'' I mean the title of a widget. You can opt to show this or not for every single widget. And in the sourcecode all the widget titles are called ''content-fragment-headers''. So i'm not talking about the header of every page or the site, but the title of every single widget. 

Hi Steven, I was looking over your answer again.. I see that you are referring to the content-fragment-header/content/footer... I'm wondering if we are talking about the same thing here...
When I say ''content-fragment-header'' I mean the title of a widget. You can opt to show this or not for every single widget. And in the sourcecode all the widget titles are called ''content-fragment-headers''. So i'm not talking about the header of every page or the site, but the title of every single widget. 

Right - the title content (in your example, "Recente blogs") can be changed by configuring the widget, and you can also specify whether it's there or not through the widget Format options. But the usage of "content-fragment-header", "content-fragment-content", "content-fragment-footer" class names is not adjustable.