.center{
  margin: auto;
  width: 900px;
  border: 20px solid blue;
  padding: 10px;
  top: 100px;
  border-radius: 15px;
  background: white;
}

div.sticky {
    position: -webkit-sticky; /* Safari */
    position: sticky;
    top: 0;
}

body {
    background-image: url("/images/background.jpg");
}

.imgcenter {
    display: block;
    margin-left: auto;
    margin-right: auto;
    width: 75%;
}

.column {
    float: left;
}

.left, .right {
    width: 25%;
}

.middle {
    width: 50%;
}

/* Clear floats after the columns */
.row:after {
    content: "";
    display: table;
    clear: both;
}

.swmenucenter {
display: inline-block
}

/* Remove default bullets */
ul, #tree {
	list-style-type: none;
}

/* Remove margins and padding from the parent ul */
#tree {
	margin: 0;
	padding: 0;
}

/* Style the caret/arrow */
.caret {
	cursor: pointer;
	user-select: none; /* Prevent text selection */
}

/* Create the caret/arrow with a unicode, and style it */
.caret::before {
	content: "\25B6";
	color: black;
	display: inline-block;
	margin-right: 6px;
}

/* Rotate the caret/arrow icon when clicked on (using JavaScript) */
.caret-down::before {
	transform: rotate(90deg);
}

/* Hide the nested list */
.nested {
	display: none;
}

/* Show the nested list when the user clicks on the caret/arrow (with JavaScript) */
.active {
	display: block;
}