/*Resets*/
html {
    width: 100%;
    height: 100%;

    padding: 0;
    margin: 0;
    border: 0;

    font-family: sans-serif;
}

#background-image {
    position: absolute;
    left: 0;
    top: 0;

    width: 100%;
    height: 100%;

    opacity: 0.6;

    z-index: -100;
}

#oninstall, #onupdate {
  position: absolute;
  width: 50%;
  height: 20%;
  top: 25%;
  right: 25%;
}
#oninstall {

.header h1 {
  font-size: 3.4em;
  font-weight: normal;
  text-align: center;
}

.links {
  text-align: center;
}

.links a {
    border: none;
    cursor: pointer;
    color:black;
    text-decoration: none;

    background-color: rgba(0, 147, 135, 0.5);
    font-size: 1.2em;
    opacity: 0.7;
    transition: opacity 500ms;

    padding: 10px;

}

.links a:hover {
    opacity: 1.0;
    transition: opacity 500ms;
}

.links a:focus {
    outline: none;
}

/*Resets*/
html {
    width: 100%;
    height: 100%;

    padding: 0;
    margin: 0;
    border: 0;

    font-family: sans-serif;
}

#background-image {
    position: absolute;
    left: 0;
    top: 0;

    width: 100%;
    height: 100%;

    opacity: 0.6;

    z-index: -100;
}

#oninstall, #onupdate {
  position: absolute;
  width: 50%;
  height: 20%;
  top: 25%;
  right: 25%;
}

.header h1 {
  font-size: 3.4em;
  font-weight: normal;
  text-align: center;
}

.links {
  text-align: center;
}

.links a {
    border: none;
    cursor: pointer;
    color:black;
    text-decoration: none;

    background-color: rgba(0, 147, 135, 0.5);
    font-size: 1.2em;
    opacity: 0.7;
    transition: opacity 500ms;

    padding: 10px;

}

.links a:hover {
    opacity: 1.0;
    transition: opacity 500ms;
}

.links a:focus {
    outline: none;
}

Remove useless oninstall block in CSS
Remove useless oninstall block in CSS
