I looked around in many of these xsl files, but I haven't found the place where the text alignment and color of the header is set...
The generation of these help files seems quite complicated, with more than 30 xsl files applied on it...
If someone is able to find out how to do it, it would be of great help.

Michele, you might have not seen one of the messages here, but I made the TDEUI file for digiKam, if you want to push it to GIT.
I also provided the Gimp file of the ribbon for all apps. It might be good to keep it somewhere, for future reference.
It would be good to write somewhere, probably in the ''About'' box, that some of the icons used here and in the splash are from the KFaenza icon theme.

Here is some infos about the KFaenza icon theme:
# Written by Tiheum (matthieu.james@gmail.com)
# Edited for use in Kde by snakebite

Thank you!
-Alexandre