Thanks for your answer!
ASHFAQ : it will change the line color, but not fill below.
What I did so far...
on the sheet style, pick up the pen ultimate style.
On the sheet expressions -> add an expression =15 with a daker background.
It almost works but: 1 it's not really good practice/dev. 2 the resultat is not really beautiful when the scale, size change.
Any other idea?
As the combo chart doesn't allow line and area you're best option is the transparent overlay you've used. An alternative (which will depend on the granularity of your dimension) may be to use the bar and line combo. Again this isn't an elegant solution but you'll have less layout issues than the overlay.