I decided to study the 555 fade in/out circuit, to understand its working and to take the control of the LED into my own hands...
I failed, realising I'm just a young padawan that has a lot yet to learn about electronics. Since I don't understand all the details (calculating the fade-in/fade-out times and the frequency and amount of "breathing" effect), I won't even try to explain the maths and current flows and everything. Whatever. It kinda works somehow. Meh.
I also tried getting into the simulators, but couldn't make it work in EasyEDA neither in CircuitLabs (I didn't even try to get into the more professional spice stuff, I don't understand sh*t there...). Too bad I don't study EE :)
BUT! Theory shouldn't be an obstacle to the "hacker attidute and the hands-on imperative" (Steven Levy, Hackers: Heroes of the Computer Revolution), right? I mean it still glows up/down somehow and nobody will ever know what I really wanted to achieve. For artistic purposes and for version 0.1 (AND after three full afternoons of studying theory and wnb calculating stuff) it's good enough :)
With a red LED it works like this (I didn't want red, but okay):
And with a white LED it looks like this (added 2.4k resistor across the C and E terminals of the transistor, to lift the emmiter voltage a bit- suggested by @Bharbour ,tnx for the interest and the patience to explain things to me):
I ordered some low power 555 timers, some ATtiny microcontrollers and LDRs and phototransistors. And button batteries. Maybe I'll return to this theoretical study after I have some fun with microcontrollers, or after trying some other circuits or just adapting this one to work at 3v (for coin cells). I learned a lot from this: from some kind of very basic circuit analysis, to capacitor working (and important calculations), BJT transistors and their modes of operation, LEDs...
Now it's time to go back to more known territory of computer code and take a break from all this analog wizardry :'D