How to make theme button affect whole page (HTML, CSS)

Question

I have a dark theme/light theme button and it works well but I was just wondering about how to change the background colour of the navigation bar. This is my code:

body {
  margin: 0px;
  padding: 0px;
  overflow-y: scroll;
  font-family: Helvetica;
  font-size: 18px;
  background-color: #bdbdbd;
}

.nav {
  background-color: #222;
  position: relative;
  width: 100%;
}

.nav_wrapper {
  width: 960px;
  margin: 0 auto;
  text-align: left;
}

.nav ul {
  list-style-type: none;
  padding: 0;
  margin: 0;
  position: relative;
}

.nav ul li {
  display: inline-block;
  background-color: #222;
  transition: background-color 0.3s ease-in;
  position: relative;
  padding-top: 3px;
  padding-bottom: 3px;
}

.nav ul li img {
  width: 12px;
  height: 10px;
  vertical-align: middle;
  padding-left: 10px;
}

.nav ul li a,visited {
  display: block;
  padding: 15px;
  text-decoration: none;
  transition: color 0.2s ease-in;
}

.nav ul li a:hover {
  text-decoration: none;
  color: #099;
}

.navhome {
  color: #099;
  border-bottom: 3px solid #099;
}

.navother {
  color: #ccc;
}

.content {
  text-align: center;
  line-height: 130%;
  font-size: 20px;
  display: inline-block;
}

.bdaimg {
  width: 50%;
  display: block;
  margin-left: auto;
  margin-right: auto;
  margin-bottom: 40px;
  margin-top: 30px;
  transform: scale(1);
  transition-duration: 0.3s;
  border-radius: 15px;
}

.bdaimg:hover {
  transform: scale(1.1);
  transition-duration: 0.3s;
}

.nu {
  font-weight: bold;
  font-size: 25px;
}

h3 {
  font-weight: bold;
  text-decoration: underline;
}

h2 {
  font-weight: bold;
  text-decoration: underline;
}

.bdaimg2 {
  display: block;
  margin-left: auto;
  margin-right: auto;
  margin-bottom: 40px;
  margin-top: 30px;
  transform: scale(1);
  transition-duration: 0.3s;
  border-radius: 15px;
}

.bdaimg2:hover {
  transform: scale(1.1);
  transition-duration: 0.3s;
}

/*Light Switch*/
label {
  display: block;
  height: 25px;
  width: 100px;
  background: white;
  text-align: center;
  font: 14px/25px Helvetica, Arial, sans-serif;
  margin: 20px 0;
  position: absolute;
}
 
label:hover {
  background: #ddca7e;
  color: white;
  cursor: pointer;
}
 
input#lightswitch {
   position: fixed ;
   top: -9999px;
   left: -9999px;
}

input#lightswitch + .content {
  background-color: #bdbdbd;
  transition: background-color 0.5s ease-in;
}

/*Switched Off*/
input#lightswitch:checked + .content  {
  background-color: #222;
  transition: background-color 0.5s ease-in;
}
 
input#lightswitch:checked + .content {
  color: white;
  transition: color 0.5s ease-in;
}
 
input#lightswitch:checked + .content {
  color: white;
  transition: color 0.5s ease-in;
}

<div class="nav">
  <div class="nav_wrapper">
    <ul>
      <li><a class="navother" href="Home.html">Home</a></li>
      <li><a class="navother" href="CR.html">Cultural Revolution</a></li>
      <li><a class="navhome" href="BDA.html">Beijing Dance Academy</a></li>
      <li><a class="navother" href="CNY.html">Chinese New Year</a></li>
    </ul>
  </div>
</div>
<label for="lightswitch">Light Switch</label>
<input type="checkbox" id="lightswitch" />
<div class="content">
  <img src="bda.png" alt="BDA" class="bdaimg">
<p>
  <h2>Beijing Dance Academy</h2>
  Beijing Dance Academy, or BDA, was the first professional dance school founded in China, in 1954. The academy was helped by Soviet Russia in building the academy and the techniques they used to learn dance. Although Russia helped with the Beijing Dance Academy, China's repressive Communist government forbidded Russian presence in the school. The Beijing Dance Academy soon opened back up to all people from around the world when, in 1976, China's Communist leader, Mao Zedong, died. 
  <br>
  <br>
  Several times a year, Beijing Dance Academy holds auditions to be accepted into the school, for more disadvantaged areas officals come around small towns and schools and pick out children of age, the academy initially only accepted students from ages 11 to 18 but eventually the age range expanded. They then test the student's basic body abilities. More than 2,000 children audition to be accepted into the academy but only 1 in every 20 make the cut. Majority of the children in the academy are Chinese but the academy has an additional program that allows international students to apply.
  <br>
  <br>
  <h3>
    A Day in the Life of a Student
  </h3>
  A day of a Beijing Dance Academy student starts with basic warm-ups and stretches such as flexibility exercises and drills for turns and jumps. After breakfast, the students have a ballet technique class followed by character dance or pas de deux exercises. Then after lunch, the students study normal subjects such as mathematics or Chinese for 3 hours. In the afternoon, they have repertory classes or study or do homework. The Beijing Dance Academy is run for the whole week...
</p>
<p class="nu">
  INCLUDING SATURDAY AND SUNDAY!
  <img src="sadface.png" alt="SpongebobSadFace" class="bdaimg2">
</p>
</div>

Answer 1

You can't target the styles of the nav because its before the input. If you are able to change the order of the elements so the input is before the nav and then positioned in the place you want.

<label for="lightswitch">Light Switch</label>
<input type="checkbox" id="lightswitch" />
<div class="nav">...</div>
<div class="content">...</div>

Then you add this to the css:

label { top: 50px; } //whatever position you need
input#lightswitch ~ .content {
  background-color: #bdbdbd;
  transition: background-color 0.5s ease-in;
}

/*Switched Off*/
input#lightswitch:checked ~ .content  {
  background-color: #222;
  transition: background-color 0.5s ease-in;
}

input#lightswitch:checked ~ .content {
  color: white;
  transition: color 0.5s ease-in;
}

input#lightswitch:checked ~ .nav {
  background-color: #bdbdbd; //whatever color you need
  transition: background-color 0.5s ease-in;
}
input#lightswitch:checked ~ .nav .nav_wrapper ul li {
  background-color: #bdbdbd; 
  transition: background-color 0.5s ease-in;
}

fiddle