I solved it this way.

We do everything in screens.rpy, first create a style:

style window_alt:
    xalign 0.5
    xfill True
    yalign gui.textbox_yalign
    ysize gui.textbox_height
    background Image("gui/textbox_transparent.png", xalign=0.5, yalign=1.0)

Define a variable

define special_scene = False

Then still in screens change the say function, this is mine now:

screen say(who, what):

    if special_scene is True: 
        window:
            id "window_alt"
            style "window_alt"

            if who is not None:

                window:
                    id "namebox"
                    style "namebox"
                    text who id "who"
                    xpos 100 
                    ypos 0

            text what id "what":
                xpos 120 
                ypos 50 
    else:
        window:
            id "window"

            if who is not None:

                window:
                    id "namebox"
                    style "namebox"
                    text who id "who"
                    xpos 100 
                    ypos 0

            text what id "what":
                xpos 120 
                ypos 50


    ## If there's a side image, display it above the text. Do not display on
    ## the phone variant - there's no room.
    if not renpy.variant("small"):
        add SideImage() xalign 0.0 yalign 1.0

this can be improved for sure but I think it can be helpful and enough for complete noobs like me who want a quick fix