dots/usr/share/themes/Dracula/gtk-3.0/widgets/column-header.css

/*****************
 * column-header *
 *****************/
column-header .button,
column-header .button:active {
    padding: 3px 8px;
    border-width: 0 1px 1px 0px;
    border-radius: 0;
    border-style: solid;
}

column-header .button,
column-header .button:active,
column-header .button:focus,
column-header .button:active:focus {
    border-color: mix(@base_color, black, 0.2);
    border-bottom-color: mix(@base_color, black, 0.2);
    background-color: shade(@theme_base_color, 0.97);
    background-image: none;
}

column-header .button:hover,
column-header .button:active:hover,
column-header .button:hover:focus,
column-header .button:active:hover:focus {
    border-color: mix(@base_color, black, 0.2);
    background-color: shade(@theme_base_color, 0.99);
    background-image: none;
    color: @theme_selected_bg_color;
}

column-header:last-child .button {
    border-width: 0 0 1px 0;
}