Using this script with TamperMonkey you can have your whatsapp web turned into an un-noticeable dark theme app

Dark Theme Whatsapp Web.user.js

// ==UserScript==
// @name          Dark Theme Whatsapp Web
// @namespace     http://userstyles.org
// @description	  My custom theme for Whatsapp Web. With orange and dark shades.
// @author        David Jiménez
// @homepage      https://userstyles.org/styles/109575
// @include       https://web.whatsapp.com/
// @run-at        document-start
// @version       0.20170512183823
// ==/UserScript==
(function() {var css = [
    ".tail-container {",
    "    display: none",
    "}",
    "span.message-datetime {",
    "    color: dimgray",
    "}",
    "emojitext {",
    "    color: gray",
    "}",
    "message-text quoted-msg-text {",
    "    color: gray",
    "}",
    "span.emojitext {",
    "   color: gray",
    "}",
    ".link-preview-body {",
    "    background-color: #181818",
    "}",
    ".bubble {",
    "    background-color: #181818",
    "}",
	".intro-image {",
	"    background-image: url(https://lh6.googleusercontent.com/-ri8vZ-Dx2TQ/VMDcZLCLNcI/AAAAAAAACGc/vnQqkc2fr9A/w356-h355-no/intro-connection.png)!important;",
	"}",
	".pane-chat-body,",
	".pane-intro,",
	".chatlist .chat,",
	".chatlist .location,",
	"#startup,",
	".pane-list-body {",
	"    background: #181818!important;",
	"}",
	".app-wrapper::before,",
	".app-wrapper {",
	"    background: #0a0a0a !important;",
	"}",
	"::-webkit-scrollbar-track {",
	"    background: #383838 !important;",
	"}",
	"::-webkit-scrollbar-thumb {",
	"    background-color: #484848 !important;",
	"}",
	".drawer-body::-webkit-scrollbar {",
	"    width: 10px!important;",
	"}",
	".pane-intro:after {",
	"    border-top: 6px solid orange;",
	"}",
	".app.three {",
	"    background: black;",
	"}",
	".drawer-body .drawer-section-photo {",
	"    background: none!important;",
	"    margin-top: -20px;",
	"    margin-bottom: -20px;",
	"}",
	".chatlist .active.location,",
	".chatlist .chat.active {",
	"    border-bottom: 2px solid #ff8000 !important;",
	"}",
	".chatlist .active.location:hover,",
	".chatlist .chat.active:hover {",
	"    border-bottom: 2px solid #383838 !important;",
	"}",
	".chatlist .active.location:hover:before,",
	".chatlist .chat.active:hover:before,",
	".chatlist .active.location:before,",
	".chatlist .chat.active:before {",
	"    background-color: black !important;",
	"}",
	".chatlist .chat:hover,",
	".chatlist .location:hover {",
	"    border-bottom: 1px solid #333 !important;",
	"}",
	".chatlist .chat:hover:before,",
	".chatlist .location:hover:before,",
	".infinite-list.chatlist {",
	"    background-color: #282828 !important;",
	"}",
	".dropdown {",
	"    box-shadow: 0px !important;",
	"    background-color: #222 !important;",
	"}",
	".dropdown .menu-item a:hover {",
	"    background-color: #333;",
	"    color: white;",
	"}",
	".dropdown .menu-item a {",
	"    color: white;",
	"}",
	".pane-chat-header,",
	".pane-header,",
	".drawer-body {",
	"    background-color: black !important;",
	"}",
	".chatlist .chat:hover,",
	".chatlist .location:hover,",
	".infinite-list-item:hover {",
	"    background: #181818 !important;",
	"}",
	".active.location,",
	".chat.active:hover,",
	".active.location:hover,",
	".chat.active {",
	"    background: #0a0a0a !important;",
	"}",
	".infinite-list-item .list-title {",
	"    background-color: black!important;",
	"}",
	".subheader-search.active {",
	"    border: none!important;",
	"    box-shadow: none!important;",
	"}",
	".chat-title span,",
	".chat-time,",
	".drawer-section-title,",
	".msg-search-notfound {",
	"    color: white!important;",
	"}",
	".block-compose,",
	".drawer-section,",
	".drawer-section-expand {",
	"    background-color: #616161!important;",
	"}",
	".input-container .input {",
	"    color: #f5f5f5!important;",
	"}",
	".menu-tabs-emoji {",
	"    background-color: #A0C8CD!important;",
	"}",
	".media-panel {",
	"    background-color: rgba(35, 35, 35, 0.95)!important;",
	"}",
	".msg-unread-body,",
	".media-panel-header {",
	"    background-color: #9E9C9C!important;",
	"}",
	".infinite-list-item .list-title {",
	"    border-bottom: 1px solid #616161!important;",
	"}",
	".chatlist .chat:after,",
	".chatlist .list-title:after {",
	"    border-bottom: none!important;",
	"}",
	".chat:hover:not(.active),",
	".location:hover:not(.active) {",
	"    background-color: inherit!important;",
	"}",
	".block-compose .input-container,",
	".cont-input-search,",
	".input-search,",
	".pane-subheader {",
	"    background-color: #282828 !important;",
	"    color: white;",
	"}",
	".cont-input-search {",
	"    border: 1px solid #0a0a0a;",
	"}",
	".drawer-section-title .col-side {",
	"    color: black!important;",
	"}",
	".cont-input-search .input-placeholder,",
	".chat-body .ellipsify,",
	".chatlist .is-unread .chat-status,",
	".chatlist .is-unread .header-secondary,",
	".chatlist .is-unread .location-secondary,",
	".chatlist .unread .chat-status,",
	".chatlist .unread .header-secondary,",
	".chatlist .unread .location-secondary,",
	".drawer-value {",
	"    color: #c2c2c2 !important;",
	"}",
	".infinite-list-item .ellipsify,",
	".infinite-list-item .emojitext {",
	"    color: #ff8000;",
	"}",
	".pane-list-header,",
	".pane-header,",
	".drawer-header {",
	"    background-color: #222 !important;",
	"}",
	".menu-tabs-emoji {",
	"    background-color: #333!important;",
	"}",
	".input-container .input {",
	"    color: #f5f5f5!important;",
	"    background: #222 !important;",
	"}",
	".emoji-search-row-input>div {",
	"    background: #222 !important;",
	"}",
	".block-compose,",
	".drawer-section,",
	".drawer-section-expand {",
	"    background-color: #333!important;",
	"}",
	".block-compose .input-container,",
	".cont-input-search,",
	".input-search,",
	".pane-subheader {",
	"    background-color: #222!important;",
	"    color: white;",
	"}",
	".block-compose .input-container {",
	"    border-color: #444;",
	"}",
	".intro {",
	"    background-color: #222;",
	"}",
	".chat-status,",
	".header-secondary {",
	"    color: #fafafa !important;",
	"}",
	".pane-chat-footer {",
	"border-left: 0px;",
	"}",
	"",
	".chatlist .chat-body, .chatlist .header-body {",
	"  border-top: 0px;",
	"}"
].join("\n");
if (typeof GM_addStyle != "undefined") {
	GM_addStyle(css);
} else if (typeof PRO_addStyle != "undefined") {
	PRO_addStyle(css);
} else if (typeof addStyle != "undefined") {
	addStyle(css);
} else {
	var node = document.createElement("style");
	node.type = "text/css";
	node.appendChild(document.createTextNode(css));
	var heads = document.getElementsByTagName("head");
	if (heads.length > 0) {
		heads[0].appendChild(node);
	} else {
		// no head yet, stick it whereever
		document.documentElement.appendChild(node);
	}
}
})();

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