Hello Jonas,
I had a look into the standard CSS of the crosstab and there are a lot of attributes and dependancies involved. Your CSS selector is probably not powerful enough. You can find a working example of word-wrap on a table here: http://jsfiddle.net/j08691/FcVSc/
I would advise that you use the developer tools of your browser (available through F12 for example) to play with the CSS selector of the crosstab to pin point the right setup that actually breaks your header title and save it to your custom css file. You will find a structure like this:
You will notice that the class you are trying to override targets the column headers (first table tag) but there is more on the DOM-tree to consider and to check to accomplish the line break.
Regards
Martin