In: Computer Science
Here is the HTML code for UI. The UI is made with an assumption of hospital's website
<!DOCTYPE html>
<html>
<head>
        <title>Home work</title>
        <script src="theme_changer.js"></script>
        <link rel="stylesheet" type="text/css" href="theme 1.css" id="theme">
</head>
<body>
        <header>XYZ Clinic</header>
        <nav><a href="#">Home</a> | <a href="#">Contact Us</a> | <a href="#" onclick="change_theme()">Change Theme</a></nav>
        <section id="main_section">
                
                <aside>
                        we are delivering the quality services in the filed of hospitality and helth care.
                </aside>
                <article id="first_article">
                        <figure>
                                <h1>Cardiologist</h1>
                                <img src="image1.jpg"  style="width:30%">
                                <figcaption>Dr. John</figcaption>
                        </figure>
                </article>
                <article id="second_article">
                        <figure>
                                <h1>Neurologist</h1>
                                <img src="image2.jpg"  style="width:30%">
                                <figcaption>Dr. Alberto</figcaption>
                        </figure>
                </article>
                <article id="third_article">
                        <figure>
                                <h1>Dentist</h1>
                                <img src="image3.jpg"  style="width:30%">
                                <figcaption>Dr. Marita</figcaption>
                        </figure>
                </article>
                <footer>Thank you</footer>
        </section>
</body>
</html>
Explaination:
as mantioned we have putted
along with external stylesheet and javascript.
Here is the them 1.css
header{
        text-align: center;
        background-color: #ccbbff;      
}
nav > a{
        text-decoration: none;
        color: white;
}
nav{
        background-color: #981267;
        color: white;
}
aside {
        width: 30%;
        float: right;
        background-color: yellow;
        font-style: italic;
        margin-left: 15px;
        padding-left: 15px;
}
#first_article{
        background-color: #982134;
}
#second_article{
        background-color: #986734;
}
#third_article{
        background-color: #981267;
}
figure{
        align-content: center;
}
section{
        background-color: #345677;
}
footer{
        text-align: center;
}
and the code for theme 2.css
header{
        text-align: center;
        background-color: #ccbbff;      
}
nav > a{
        text-decoration: none;
        color: white;
}
nav{
        background-color: #981267;
        color: white;
}
aside {
        width: 30%;
        float: right;
        background-color: yellow;
        font-style: italic;
        margin-left: 15px;
        padding-left: 15px;
}
#first_article{
        background-color: #986734;
}
#second_article{
        background-color: #981267;
}
#third_article{
        background-color: #982134;
}
figure{
        align-content: center;
}
section{
        background-color: #345677;
}
footer{
        text-align: center;
}
and here is the final functional code for changing them using Javascript theme_changer.js with explanation in with it.
//Fisr we have Define a function which will handle my themes 
//Lets consider it change_theme()
function change_theme() {
        //Now we have to find the link element in wich we have define our style sheet.
        element = document.getElementById("theme");
        //After that we have to check that which theme or style sheet our web page is using for that we will use the Ternary operator 
        //If our web page is using theme 1 then the variable elemet.p will be theme 2
        //If web page is using theme 2 the the variable elemet.p will carry the value theme 1
        element.p = 'theme 2' == element.p ? 'theme 1' : 'theme 2';
        // After that we have to set alternate style sheet to our web page 
        // For that we will use the value stored in elemet.p and append .css in it
        // and finaly assign the value to href atribute of link elemet.
        element.href = element.p + '.css';
}