A black line in a rectangle

Hi

I want to create a transparent rectangle in the form view

I create a formula

I enter : html("<hr>")

I get a rectangle with a blalck line inside

I go to "style" on RGB

I select "background color" and enter 3 times "238"

I save

I go to the form view

And the black line is always there…

Where is the mistake ?

Thank you.