THANKS VERY MUCH FOR ALL TIPS
They are warmly welcome.

I tried Theme maker, it's easy to use, but to make a complete theme from scratch would be a Titan work ! For now, i just stick to modifing Nuvo Pearl ...

One thing among all I would love to do is to find a way to make the background of GPE Summary completely transparent, and have white text for the appointments/todos. Anybody knows how to make that ?

Is it possible (an legal) to get the PNG from stock themes that comes with the OS ? I like very much the default Echo and would like to use is as a base.