Add Dropdown Menu in Blogger Website

Here are all codes I found from Internet to display a dropdown menu on my 91sec.blogspot.com website.

1 Add the following code to Template > Customize > Advanced > Add CSS.
This will place the code in Template > Edit HTML.

/* ----- START DROPDOWN CSS ----- */

#navigationbar {
width: 100%; /* change the width of the navigation bar */
height: 40px; /* change the height of the navigation bar */
}

#navigationcss {
margin: 0 auto;
padding: 0;
}

#navigationcss ul {
float: none;
list-style: none;
margin: 0;
padding: 0;
overflow: visible;
}

#navigationcss li a, #navigationcss li a:link, #navigationcss li a:visited {
color: #ffffff; /* change color of the main links */
display: block;
margin: 0;
padding: 10px 30px; /* change the first number for the top/bottom spacing, and the second number for left/right spacing */
}

#navigationcss li a:hover, #navigationcss li a:active {
color: #69b4ff ; /* change the color of the links when hovered over */
margin: 0;
padding: 10px 30px; /* make sure these are the same as the section above! */
}

#navigationcss li li a, #navigationcss li li a:link, #navigationcss li li a:visited {
background: #ffffff; /* change the background color of the drop down box */
width: 150px;
color: #000000; /* change the color of the drop down links */
float: none;
margin: 0;
padding: 7px 10px; /* similar to above, change for the spacing around the links */
}

#navigationcss li li a:hover, #navigationcss li li a:active {
background: #69b4ff ; /* change the background color of drop down items on hover */
color: #ffffff; /* change the color of drop down links on hover */
padding: 7px 10px; /* keep these the same as the above section */
}

#navigationcss li {
float: none;
display: inline-block;
list-style: none;
margin: 0;
padding: 0;
}

#navigationcss li ul {
z-index: 9999;
position: absolute;
left: -999em;
height: auto;
width: 150px;
margin: 0;
padding: 0;
}

#navigationcss li:hover ul, #navigationcss li li:hover ul, #navigationcss li li li:hover ul, #navigationcss li.sfhover ul, #navigationcss li li.sfhover ul, #navigationcss li li li.sfhover ul {
left: auto;
}

/* ----- END DROPDOWN CSS ----- */

.tabs-outer, .tabs .widget ul {overflow: visible;}

]]></b:skin>

Note: Code ".tabs-outer, .tabs .widget ul {overflow: visible;}" is used to resolve the issue sometimes drop down menu invisible on some blogger template.

2. Go to Layout > Add a new HTML/Javascript Gadget and add the following code to it.
Different blogger template may have different layout to place this Gadget.

<div id="navigationbar">
<ul id='navigationcss'>
<li><a href="http://91sec.blogspot.com">Home</a></li>

<li><a href="https://91sec.blogspot.ca/p/sitemap2.html">Sitemap</a>
<ul>
<li><a href='https://91sec.blogspot.ca/p/sitemap2.html'>Sitemap Page</a></li>
<li><a href='https://91sec.blogspot.ca/p/sitemap_2.html'>Categories</a></li>
</ul>
</li>

<li><a href="">Resources</a>
<ul>
<li><a href='https://91sec.blogspot.ca/p/othe.html'>Online Resources</a></li>
<li><a href='https://91sec.blogspot.ca/p/new-things-to-learn.html'>Learning</a></li>
</ul>
</li>

<li><a href="">Tools</a>
<ul>
<li><a href='http://91sec.blogspot.com/p/tools.html'>Network Tools</a></li>
<li><a href='http://91sec.blogspot.com/2015/07/my-top-network-security-tools.html'>Security Tools</a></li>
<li><a href='http://91sec.blogspot.com/2013/08/some-of-my-network-troubleshooting-tools.html'>Internet Tools</a></li>
<li><a href='http://91sec.blogspot.com/2016/03/collection-of-website-and-tools-for.html'>Blogger Tools</a></li>
</ul>
</li>

<li><a href="">About</a>
<ul>
<li><a href='https://91sec.blogspot.ca/p/about-me.html'>About Me</a></li>
<li><a href='https://91sec.blogspot.ca/p/consulting-services.html'>Consulting</a></li>
<li><a href='https://91sec.blogspot.ca/p/services.html'>Services</a></li>
</ul>
</li>

<li><a href="http://www.51sec.org">51sec.org</a></li>
</ul>
</div>

Another CSS example to add a drop down menu into Blogspot:
https://icanbuildablog.com/2015/08/how-to-add-a-drop-down-menu-to-a-blogger-blog-without-javascript/

/* DROPDOWN MENU BY icanbuildablog.com */
.tabs-inner .widget ul#icbabdrop {
  text-align: left;
  display: inline;
  margin: 0;
  padding: 15px 4px 17px 0;
  list-style: none;
  border:none;
}
.tabs-inner .widget ul#icbabdrop li {
  font-size: 12px/18px;
  font-family: sans-serif; /* Font for the menu */
  display: inline-block;
  margin-right: -4px;
  position: relative;
  padding: 15px 20px;
  background: #fff; /* background colour of the main menu */
  float:none;
  cursor: pointer;
  -webkit-transition: all 0.2s;
  -moz-transition: all 0.2s;
  -ms-transition: all 0.2s;
  -o-transition: all 0.2s;
  transition: all 0.2s;
}
.tabs-inner .widget ul#icbabdrop li a {
padding:0;
font-family: sans-serif; /* Font for the menu links */
border:0;
}
.tabs-inner .widget ul#icbabdrop li:hover {
  background: #555; /* background colour when you roll over a menu title */
  color: #fff; /* font colour when you roll over a menu title */ 
}
.tabs-inner .widget ul#icbabdrop li:hover a {
  background: transparent;
  color: #fff; /* font colour when you roll over a menu title link */ 
}
.tabs-inner .widget ul#icbabdrop li ul {
  z-index:1000; 
  border:none;
  padding: 0;
  position: absolute;
  top: 45px;
  left: 30px;
  float:none;
  width: 150px;
  -webkit-box-shadow: none;
  -moz-box-shadow: none;
  box-shadow: none;
  display: none;
  opacity: 0;
  visibility: hidden;
  -webkit-transiton: opacity 0.2s;
  -moz-transition: opacity 0.2s;
  -ms-transition: opacity 0.2s;
  -o-transition: opacity 0.2s;
  -transition: opacity 0.2s;
}
.tabs-inner .widget ul#icbabdrop li ul li { 
  background: #555; /* background colour of the sub menu items */
  display: block; 
  color: #fff; /* font colour of the sub menu items */
  text-shadow: 0 -1px 0 #000;
}
ul#icbabdrop li ul li a{
  color:#fff  /* link colour of the sub menu items */
}
.tabs-inner .widget ul#icbabdrop li ul li:hover { 
  background: #666; /* background colour when you roll over sub menu items */
}
.tabs-inner .widget ul#icbabdrop li:hover ul {
  display: block;
  opacity: 1;
  visibility: visible;
}

Here is html code to put menu into Blog:

<center> <ul id="icbabdrop">
  <li><a href="#">Home</a></li>
  <li><a href="#">About</a></li>
  <li>
    Topics
    <ul>
      <li><a href="#">Lifestyle</a></li>
      <li><a href="#">Blogging</a></li>
      <li><a href="#">Art and Design</a></li>
    </ul>
  </li>
<li>
    Topics 2
    <ul>
      <li><a href="#">Lifestyle 2</a></li>
      <li><a href="#">Blogging 2</a></li>
      <li><a href="#">Art and Design 2</a></li>
    </ul>
  </li>
 <li><a href="#">Portfolio</a></li>
  <li><a href="#">Contact</a></li>
</ul> </center>