Partialy responding to myself:

"Alt+F2" then typing "kdesu kate" works well!

If I well understand, the "things" displayed with "root" privileges are 
displayed with "smoothed" fonts. With Kate started as described above, 
the characters is well displayed. Hummmm: the characteres displayed in 
the TEXTAREA of Firefox 3.6.20 (initial) are well displayed, too.

Sincerly,
Patrick