// reset

html, body, div, span, object, iframe,
h1, h2, h3, h4, h5, h6, p, blockquote, pre,
abbr, address, cite, code,
del, dfn, em, img, ins, kbd, q, samp,
small, strong, sub, sup, var,
b, i,
dl, dt, dd, ol, ul, li,
fieldset, form, label, legend,
table, caption, tbody, tfoot, thead, tr, th, td,
article, aside, canvas, details, figcaption, figure,
footer, header, hgroup, menu, nav, section, summary,
time, mark, audio, video {
    margin:0;
    padding:0;
    border:0;
    outline:0;
    font-size:100%;
    vertical-align:baseline;
    background:transparent;
}
html,body{
  height: 100%;
  padding: 0;
  margin: 0;
  line-height:1;
}

article,aside,details,figcaption,figure,
footer,header,hgroup,menu,nav,section {
    display:block;
}

nav ul {
    list-style:none;
}

blockquote, q {
    quotes:none;
}

blockquote:before, blockquote:after,
q:before, q:after {
    content:'';
    content:none;
}

a {
    margin:0;
    padding:0;
    font-size:100%;
    vertical-align:baseline;
    background:transparent;
}

ins {
    text-decoration:none;
}

mark {
    font-style:italic;
    font-weight:bold;
}

del {
    text-decoration: line-through;
}

abbr[title], dfn[title] {
    border-bottom:1px dotted;
    cursor:help;
}

table {
    border-collapse:collapse;
    border-spacing:0;
}

hr {
    display:block;
    height:1px;
    border:0;
    border-top:1px solid #cccccc;
    margin:1em 0;
    padding:0;
}

input, select {
    vertical-align:middle;
}

// end reset begin skelet

article{
  min-height: calc(100% - 150px);
  background: #123; // demostyle
}
header{
  height: 50px;
  background: gray; // demostyle
}
footer{
  height: 150px;
  background: red; // demostyle
}
.js-popup{
  display: none;
}
.popup-background{
  position: fixed;
  background: #000;
  opacity: .8;
  top: 0;
  right: 0;
  bottom: 0;
  left: 0;
}
.popup-content{
  width: 300px;
  min-height: 200px;
  max-width: 100%;
  background: blue; // demostyle
  position: fixed;
  left: 50%;
  top: 50%;
  transform: translate(-50%, -50%);
}

// your code here