index.css

/* reset */
@import url('https://fonts.googleapis.com/css2?family=Noto+Sans+JP&family=Roboto&display=swap');
body, p, a, input, button h1, h2, h3, h4, h5, h6{
  color: #292929;
	font-family: 'Roboto', 'Noto Sans JP', sans-serif;
}

a, button {
	text-decoration: none;
	color: unset;
  transition: 0.2s;
}

a:hover {
  opacity: 0.6;
  transition: 0.2s;
  cursor: pointer;
}

/* basis */
body {
  background: #F0F2F5;
  margin: 0;
  padding-top: 75px;
  padding-left: 20px;
  padding-right: 20px;
  padding-bottom: 80px;
  box-sizing: border-box;
  min-height: 100vh;
	overflow-x: hidden;
}

header {
  color: #24292F;
  background: #FFFFFF;
  box-shadow: 0 1px 4px 0 rgba(0, 0, 0, 0.24);
  display: flex;
  align-items: center;
  justify-content: space-between;
  position: fixed;
	top: 0;
  left: 0;
  width: 100%;
  height: 55px;
  line-height: 20px;
  padding-top: 18px;
  padding-left: 20px;
  padding-right: 20px;
  padding-bottom: 14px;
  box-sizing: border-box;
}

header span{
  max-width: 60%;
  text-align: right;
}

.logo {
  color: #24292F;
  text-decoration: none;
  display: flex;
  align-items: center;
  justify-content: center;
}

.logo i{
  padding-right: 5px;
  font-size: 28px;
}

.logo h1 {
  font-size: 22px;
}

main {
  min-height: calc(100vh - 200px);
}

footer{
  color: rgba(0, 0, 0, 0.54); 
	border-top: 1px solid rgba(0, 0, 0, 0.24);
	display: flex;
	align-items: center;
  justify-content: center;
  height: 60px;
  width: calc(100vw - 40px);
  margin-top: 20px;
  margin-bottom: -80px;
}

@media screen and (min-width:840px) {
  body, header {
    padding-left: calc((100vw - 800px) / 2);
    padding-right: calc((100vw - 800px) / 2);
  }
  footer {
    width: 800px;
  }
}

/* main */
article h2{
  font-size: 20px;
  font-weight: bold;
  margin-top: 35px;
  margin-bottom: 25px;
}

button {
  background-image: linear-gradient(180deg, #f6f8fa, #ebf0f4 90%);
  border-radius: 6px;
  padding: 5px 15px;
  box-sizing: border-box;
  border: 1px solid rgba(27,31,36,0.25);
}

button:hover {
  cursor: pointer;
  color: #595959;
  border-color: #ccd1d5;
}

button:active {
  background: #ebf0f4;
}

input {
  width: 100%;
  padding: 10px;
  box-sizing: border-box;
  border-radius: 6px;
  border: 1px solid rgba(27,31,36,0.25);
  margin-bottom: 13px;
}