Hello Jonas,
I had a look into the standard css of the crosstab and there are a lot of css attributes involved. I think your selector is not powerful enough. You can find a working example of word-wrap in a table here: Edit fiddle - JSFiddle
I would advice that you use the developer tools of your favorite browser (available through F12) to play with the CSS settings of the crosstab to pin point the right setup to actually break you header title. You will find a structure like this:
You will notice that the class your trying to override targets the column headers but there is more to check.
Regards
Martin