The about window is a 450 x 250 window created by class.action.about.php styling is in class.windbinder.php specifically line 150

    public function drawRect($parent, $xPos, $yPos, $width, $height, $color = 15790320, $filled = true)
    {
        return $this->callWinBinder('wb_draw_rect', array($parent, $xPos, $yPos, $width, $height, $color, $filled));
    }

You must use decimal when working with colors such as https://convertingcolors.com/decimal-color-15790320.html
The hex value for the background is #f0f0f0