1.Install Remodal

git clone https://github.com/vodkabears/Remodal.git
cd Remodal/dist
ls
remodal.css  remodal-default-theme.css  remodal.js  remodal.min.js

cp remodal.min.js $GHOST_HOME/content/themes/casper/assets/js/
cp remodal.css $GHOST_HOME/content/themes/casper/assets/css/
cp remodal-default-theme.css $GHOST_HOME/content/themes/casper/assets/css/

mkdir -p content/themes/casper/assets/icons
cp $GHOST_HOME/versions/3.0.3/core/built/assets/icons/search.svg content/themes/casper/assets/icons/

2.Install GhostFinder

git clone https://github.com/electronthemes/ghost-finder.git
cd ghost-finder/dist
ls 
ghost-finder.js

cp ghost-finder.js $GHOST_HOME/content/themes/casper/assets/js/

3.Use Remodal and GhostFinder in Ghost template

vi /home/martin/Desktop/ghostdev/content/themes/casper/default.hbs

    {{!-- Styles'n'Scripts --}}
    <link rel="stylesheet" type="text/css" href="{{asset "built/screen.css"}}" />
    <link rel="stylesheet" type="text/css" href="{{asset "css/remodal.css"}}" />
    <link rel="stylesheet" type="text/css" href="{{asset "css/remodal-default-theme.css"}}" />
    <style>
        .remodal-search{
            height: 16px!important;
            width: 16px!important;
        }
    </style>
    {{!-- This tag outputs SEO meta+structured data and other important settings --}}
    {{ghost_head}}
</head>
<body class="{{body_class}}">
    <div class="remodal-bg">
    </div>

    <div class="remodal" data-remodal-id="modal">
        <button data-remodal-action="close" class="remodal-close"></button>
        <div>
            <img class="remodal-search" src="{{asset "icons/search.svg"}}">
            <input id="search-input" type="text" placeholder="Search a post" autofocus />
            <div id="search-result"></div>
        </div>
    </div>
    <div class="site-wrapper">
......
......
    {{ghost_foot}}
    <script src="{{asset "js/remodal.min.js"}}"></script>
    <script src="{{asset "js/ghost-finder.js"}}"></script>
    {{!-- the contentApiKey is copied from the GhostFinder customized integration--}}
    <script>
        new GhostFinder({
            input: '#search-input',
            showResult: '#search-result',
            contentApiKey: '8e698355da81e3502ebaaa24e3' 
        })
    </script>
</body>

4.Add a Search Link in Home Page

Go to http://localhost:2368/ghost/#/settings/design, add a Navigation as:
Label: Search
URL: #modal

5.Restart Ghost to take effect

Click the "Search" link, will pop up a modal window, need further customize the pop-up window to show the search box.