diff --git a/doc/source/themes/nature_with_gtoc/layout.html b/doc/source/themes/nature_with_gtoc/layout.html
deleted file mode 100644
index 6e7d8ece35133..0000000000000
--- a/doc/source/themes/nature_with_gtoc/layout.html
+++ /dev/null
@@ -1,108 +0,0 @@
-{#
-
-Subset of agogo theme
-agogo/layout.html
-
-Sphinx layout template for the agogo theme, originally written
-by Andi Albrecht.
-
-:copyright: Copyright 2007-2011 by the Sphinx team, see AUTHORS.
-:license: BSD, see LICENSE for details.
-#}
-{% extends "basic/layout.html" %}
-
-{%- block content %}
-<div class="content-wrapper">
-    <div class="content">
-        <div class="document">
-            <div class="sphinxsidebar">
-                {%- block sidebar1 %}
-                {%- block sidebartoc %}
-                <h3>{{ _('Table Of Contents') }}</h3>
-                {{ toctree(includehidden=True) }}
-                {%- endblock %}
-                {%- block sidebarsearch %}
-                <h3 style="margin-top: 1.5em;">{{ _('Search') }}</h3>
-
-                <form class="search" action="{{ pathto('search') }}" method="get">
-                    <input type="text" name="q" size="18"/>
-                    <input type="submit" value="{{ _('Go') }}"/>
-                    <input type="hidden" name="check_keywords" value="yes"/>
-                    <input type="hidden" name="area" value="default"/>
-                </form>
-                <p class="searchtip" style="font-size: 90%">
-                    {{ _('Enter search terms or a module, class or function name.') }}
-                </p>
-
-            </div>
-            {%- endblock %}
-            {# possible location for sidebar #} {% endblock %}
-
-
-            {%- block document %}
-            <div class="documentwrapper">
-                {%- if render_sidebar %}
-                <div class="bodywrapper">
-                    {%- endif %}
-                    <div class="body">
-                        {% block body %} {% endblock %}
-                    </div>
-                    {%- if render_sidebar %}
-                </div>
-                {%- endif %}
-            </div>
-            {%- endblock %}
-
-            {%- block sidebar2 %}
-
-            {% endblock %}
-            <div class="clearer"></div>
-        </div>
-    </div>
-</div>
-{%- endblock %}
-
-{%- block footer %}
-<style type="text/css">
-    .scrollToTop {
-        text-align: center;
-        font-weight: bold;
-        position: fixed;
-        bottom: 60px;
-        right: 40px;
-        display: none;
-    }
-</style>
-<a href="#" class="scrollToTop">Scroll To Top</a>
-<script type="text/javascript">
-$(document).ready(function() {
-    //Check to see if the window is top if not then display button
-    $(window).scroll(function() {
-        if ($(this).scrollTop() > 200) {
-            $('.scrollToTop').fadeIn();
-        } else {
-            $('.scrollToTop').fadeOut();
-        }
-    });
-
-    //Click event to scroll to top
-    $('.scrollToTop').click(function() {
-        $('html, body').animate({
-            scrollTop: 0
-        }, 500);
-        return false;
-    });
-});
-</script>
-
-<!-- Google Analytics -->
-<script>
-window.ga=window.ga||function(){(ga.q=ga.q||[]).push(arguments)};ga.l=+new Date;
-ga('create', 'UA-27880019-2', 'auto');
-ga('set', 'anonymizeIp', true);
-ga('send', 'pageview');
-</script>
-<script async src='https://www.google-analytics.com/analytics.js'></script>
-<!-- End Google Analytics -->
-
-{% endblock %}
diff --git a/doc/source/themes/nature_with_gtoc/static/nature.css_t b/doc/source/themes/nature_with_gtoc/static/nature.css_t
deleted file mode 100644
index 4571d97ec50ba..0000000000000
--- a/doc/source/themes/nature_with_gtoc/static/nature.css_t
+++ /dev/null
@@ -1,356 +0,0 @@
-/*
- * nature.css_t
- * ~~~~~~~~~~~~
- *
- * Sphinx stylesheet -- nature theme.
- *
- * :copyright: Copyright 2007-2011 by the Sphinx team, see AUTHORS.
- * :license: BSD, see LICENSE for details.
- *
- */
-
-@import url("basic.css");
-
-/* -- page layout ----------------------------------------------------------- */
-
-body {
-    font-family: Arial, sans-serif;
-    font-size: 100%;
-    background-color: #111;
-    color: #555;
-    margin: 0;
-    padding: 0;
-}
-
-
-div.documentwrapper {
-  width:  100%;
-}
-
-div.bodywrapper {
-/* ugly hack, probably not attractive with other font size for re*/
-    margin: 0 0 0 {{ theme_sidebarwidth|toint}}px;
-    min-width: 540px;
-    max-width: 800px;
-}
-
-
-hr {
-    border: 1px solid #B1B4B6;
-}
-
-div.document {
-    background-color: #eee;
-}
-
-div.body {
-    background-color: #ffffff;
-    color: #3E4349;
-    padding: 0 30px 30px 30px;
-    font-size: 0.9em;
-}
-
-div.footer {
-    color: #555;
-    width: 100%;
-    padding: 13px 0;
-    text-align: center;
-    font-size: 75%;
-}
-
-div.footer a {
-    color: #444;
-    text-decoration: underline;
-}
-
-div.related {
-    background-color: #6BA81E;
-    line-height: 32px;
-    color: #fff;
-    text-shadow: 0px 1px 0 #444;
-    font-size: 0.9em;
-}
-
-div.related a {
-    color: #E2F3CC;
-}
-
-div.sphinxsidebar {
-  font-size: 0.75em;
-  line-height: 1.5em;
-  width: {{ theme_sidebarwidth|toint }}px;
-  margin: 0 ;
-  float: left;
-
-  background-color: #eee;
-}
-/*
-div.sphinxsidebarwrapper{
-    padding: 20px 0;
-}
-*/
-div.sphinxsidebar h3,
-div.sphinxsidebar h4 {
-    font-family: Arial, sans-serif;
-    color: #222;
-    font-size: 1.2em;
-    font-weight: normal;
-    margin: 20px 0 0 0;
-    padding: 5px 10px;
-    background-color: #ddd;
-    text-shadow: 1px 1px 0 white
-}
-
-div.sphinxsidebar h4{
-    font-size: 1.1em;
-}
-
-div.sphinxsidebar h3 a {
-    color: #444;
-}
-
-
-div.sphinxsidebar p {
-    color: #888;
-/* padding: 5px 20px;*/
-}
-
-div.sphinxsidebar p.searchtip {
-    color: #888;
-  padding: 5px 20px;
-}
-
-
-div.sphinxsidebar p.topless {
-}
-
-div.sphinxsidebar ul {
-    margin: 10px 20px;
-    padding: 0;
-    color: #000;
-}
-
-div.sphinxsidebar a {
-    color: #444;
-}
-
-div.sphinxsidebar input {
-    border: 1px solid #ccc;
-    font-family: sans-serif;
-    font-size: 1em;
-}
-
-div.sphinxsidebar input[type=text]{
-    margin-left: 20px;
-}
-
-/* -- body styles ----------------------------------------------------------- */
-
-a {
-    color: #005B81;
-    text-decoration: none;
-}
-
-a:hover {
-    color: #E32E00;
-    text-decoration: underline;
-}
-
-div.body h1,
-div.body h2,
-div.body h3,
-div.body h4,
-div.body h5,
-div.body h6 {
-    font-family: Arial, sans-serif;
-    background-color: #BED4EB;
-    font-weight: normal;
-    color: #212224;
-    margin: 30px 0px 10px 0px;
-    padding: 5px 0 5px 10px;
-    text-shadow: 0px 1px 0 white
-}
-
-div.body h1 { border-top: 20px solid white; margin-top: 0; font-size: 200%; }
-div.body h2 { font-size: 150%; background-color: #C8D5E3; }
-div.body h3 { font-size: 120%; background-color: #D8DEE3; }
-div.body h4 { font-size: 110%; background-color: #D8DEE3; }
-div.body h5 { font-size: 100%; background-color: #D8DEE3; }
-div.body h6 { font-size: 100%; background-color: #D8DEE3; }
-
-p.rubric {
-    border-bottom: 1px solid rgb(201, 201, 201);
-}
-
-a.headerlink {
-    color: #c60f0f;
-    font-size: 0.8em;
-    padding: 0 4px 0 4px;
-    text-decoration: none;
-}
-
-a.headerlink:hover {
-    background-color: #c60f0f;
-    color: white;
-}
-
-div.body p, div.body dd, div.body li {
-    line-height: 1.5em;
-}
-
-div.admonition p.admonition-title + p, div.deprecated p {
-    display: inline;
-}
-
-div.deprecated {
-    margin-bottom: 10px;
-    margin-top: 10px;
-    padding: 7px;
-    background-color: #ffe4e4;
-    border: 1px solid #f66;
-}
-
-div.highlight{
-    background-color: white;
-}
-
-div.note {
-    background-color: #eee;
-    border: 1px solid #ccc;
-}
-
-div.seealso {
-    background-color: #ffc;
-    border: 1px solid #ff6;
-}
-
-div.topic {
-    background-color: #eee;
-}
-
-div.warning {
-    background-color: #ffe4e4;
-    border: 1px solid #f66;
-}
-
-p.admonition-title {
-    display: inline;
-}
-
-p.admonition-title:after {
-    content: ":";
-}
-
-pre {
-    padding: 10px;
-    background-color: rgb(250,250,250);
-    color: #222;
-    line-height: 1.2em;
-    border: 1px solid rgb(201,201,201);
-    font-size: 1.1em;
-    margin: 1.5em 0 1.5em 0;
-    -webkit-box-shadow: 1px 1px 1px #d8d8d8;
-    -moz-box-shadow: 1px 1px 1px #d8d8d8;
-}
-
-tt {
-    background-color: #ecf0f3;
-    color: #222;
-    /* padding: 1px 2px; */
-    font-size: 1.1em;
-    font-family: monospace;
-}
-
-.viewcode-back {
-    font-family: Arial, sans-serif;
-}
-
-div.viewcode-block:target {
-    background-color: #f4debf;
-    border-top: 1px solid #ac9;
-    border-bottom: 1px solid #ac9;
-}
-
-
-/**
- * Styling for field lists
- */
-
- /* grey highlighting of 'parameter' and 'returns' field */
-table.field-list {
-    border-collapse: separate;
-    border-spacing: 10px;
-    margin-left: 1px;
-    /* border-left: 5px solid rgb(238, 238, 238) !important; */
-}
-
-table.field-list th.field-name {
-    /* display: inline-block; */
-    padding: 1px 8px 1px 5px;
-    white-space: nowrap;
-    background-color: rgb(238, 238, 238);
-}
-
-/* italic font for parameter types */
-table.field-list td.field-body > p {
-    font-style: italic;
-}
-
-table.field-list td.field-body > p > strong {
-    font-style: normal;
-}
-
-/* reduced space around parameter description */
-td.field-body blockquote {
-    border-left: none;
-    margin: 0em 0em 0.3em;
-    padding-left: 30px;
-}
-
-// Adapted from the new Jupyter notebook style
-// https://github.com/jupyter/notebook/blob/c8841b68c4c0739bbee1291e0214771f24194079/notebook/static/notebook/less/renderedhtml.less#L59
-table {
-    margin-left: auto;
-    margin-right: auto;
-    border: none;
-    border-collapse: collapse;
-    border-spacing: 0;
-    color: @rendered_html_border_color;
-    table-layout: fixed;
-}
-thead {
-    border-bottom: 1px solid @rendered_html_border_color;
-    vertical-align: bottom;
-}
-tr, th, td {
-    vertical-align: middle;
-    padding: 0.5em 0.5em;
-    line-height: normal;
-    white-space: normal;
-    max-width: none;
-    border: none;
-}
-th {
-    font-weight: bold;
-}
-th.col_heading {
-    text-align: right;
-}
-tbody tr:nth-child(odd) {
-    background: #f5f5f5;
-}
-
-table td.data, table th.row_heading table th.col_heading {
-    font-family: monospace;
-    text-align: right;
-}
-
-
-/**
- * See also
- */
-
-div.seealso dd {
-    margin-top: 0;
-    margin-bottom: 0;
-}
diff --git a/doc/source/themes/nature_with_gtoc/theme.conf b/doc/source/themes/nature_with_gtoc/theme.conf
deleted file mode 100644
index 290a07bde8806..0000000000000
--- a/doc/source/themes/nature_with_gtoc/theme.conf
+++ /dev/null
@@ -1,7 +0,0 @@
-[theme]
-inherit = basic
-stylesheet = nature.css
-pygments_style = tango
-
-[options]
-sidebarwidth = 270