Edit: Correction. At least for my colours, the customizations weren’t lost; they just don’t seem to be being applied. I’ve said to make all the text black, but it’s still grey.
Edit edit: I’ve managed to get things working again. What I did (and I don’t know which of these bits were necessary) was go to the Customize Journal page at http://www.livejournal.com/customize/, change my style from Component to Generator, change my language from Danish (which it had peculiarly become set to when I changed the style) to English, change the style back to Component, click "Remove Customizations", and then click "Customize" and make all my changes again.