Okay, I moved the ceramic capacitor directly infront of the DHT11 sensor. I also tried to use as few as possible cables. The sensor was already conected to a dedicated ground pin on the Arduino board. Things look like getting better now. Instead of having temperature and humidity data with a difference of 2-3 when dimming the back light it's now only the humidity that has a difference of something betwenn 0.5 and 1. It's kind of hard to know as the sensor's resolution is 1.0.
Think we got near the sollution 
Anything else someone could think of what I also could try? Maybe another capacitor or something like that? Sorry for the dump question but in fact I even do not know what this little thing made of ceramic is actually doing there...
Oh, and thanks for the info about the software for the schematics. I'll take a look to each one and check if I will be able to cope with one of them.
Greez,
dialsc