electron/docs/fiddles/features/window-customization/custom-window-styles/transparent-windows/styles.css

body {
    margin: 0;
    padding: 0;
    background-color: rgba(0, 0, 0, 0); /* Transparent background */
}
.white-circle {
    width: 100px;
    height: 100px;
    background-color: white;
    border-radius: 50%;
    display: flex;
    align-items: center;
    justify-content: center;
    app-region: drag;
    user-select: none;
}