polite-popup-dark-mode.css

body .email-modal .email-modal__content {
	background-color: #191c1f;
}

body .email-modal .email-modal__top-image {
	background-color: #fff;
}

body .bottom-bar, body .bottom-bar *, body .email-modal, body .email-modal * {
	color: #fff;
}

body .email-modal .email-modal__description {
	color: #fff;
	opacity: .8;
}

body .email-modal .email-modal__input, body .bottom-bar .bottom-bar__input {
	background-color: #343a40;
	border: none;
	color: #fff;
}

body .bottom-bar .bottom-bar__close, body .bottom-bar .bottom-bar__sign-up, body .email-modal .email-modal__sign-up {
	background-color: #495057;
}

body .bottom-bar .bottom-bar__close:hover, body .bottom-bar .bottom-bar__sign-up:hover, body .email-modal .email-modal__sign-up:hover {
	background-color: #343a40;
}

body .bottom-bar {
	background-color: #191c1f;
	border-top-color: #191c1f;
}

body .bottom-bar .bottom-bar__label-text {
	color: #fff;
} 

If you paste this CSS in your website, then Polite Email Popup (politepop.com) will get a dark mode theme!