How to theme GTK OSD?

I use Mate desktop 1.26 (GTK 3.24) and I want to update my CSS theme for OSD.

Is there a way with a very simple program to keep OSD displayed to debug it with GTK-inspector? Or is someone know the CSS nodes?

The first screenshot was taken with default theme, the second with my theme. I try to remove the black border, to set the background color I use the following piece of code, but it's ridiculous:

.osd * {
    background: @theme_bg;
}

EDIT: I found an horrible way, it seems that we can't change the black border width (12px):

.osd {
    border: 12px solid @nb_border;
    border-radius: 0;
    background: @theme_bg;
}

.osd > box {
    background: @theme_bg;
    box-shadow:
        0 -11px @theme_bg, /* top */
        11px  0 @theme_bg, /* right */
        0  11px @theme_bg, /* bottom */
        -11px 0 @theme_bg, /* left */
        -11px -11px @theme_bg, /* top left */
         11px -11px @theme_bg, /* top right */
         11px  11px @theme_bg, /* bottom right */
        -11px  11px @theme_bg  /* bottom left */;
}