Issue to create a line in the formula field

Hi

I'd like to create a black line in the formula view of Ninox

I create a formula where I enter :

html("<hr color=black>")

I get a black line in the formula view but it is surraounded by a grey rectangle.

I can't erase it.

I tried :

Style

"Color" ;

RGB > 238 in each field

Or

"background color" :

RGB > 238 in each field

It does not work ! The rectangle is always there.

Can someone tell me the way to remove this grey rectangle