body {
	color: white;
}
.header {
	margin-top: 50px;		
}
.header-img {
	margin: 150, 1500, 150, 1500;
	width: 400px;
	height: 250px;
	border-radius: 4px;
}
.demo-layout-transparent { 
  background: url('../img/background.jpg') center / cover;
}
.demo-layout-transparent .mdl-layout__header,
.demo-layout-transparent .mdl-layout__drawer-button {  
  color: white;
}
.repo.mdl-card {
 margin-top: 50px; 
}
@media only screen and (min-width: 350px) {
	.header-img {
		width: 350px !important;
	}
}
.repo > .mdl-card__title {
  color: #fff;
  height: 176px;
  background: url('../img/background.jpg') center / cover;
}
.repo > .mdl-card__menu {
  color: #fff;
}
body {
	color: white;
}
.header {
	margin-top: 50px;
}
.header-img {
	margin: 150, 1500, 150, 1500;
	width: 400px;
	height: 250px;
	border-radius: 4px;
}
.demo-layout-transparent {
  background: url('../img/background.jpg') center / cover;
}
.demo-layout-transparent .mdl-layout__header,
.demo-layout-transparent .mdl-layout__drawer-button {
  color: white;
}
.repo.mdl-card {
 margin-top: 50px;
}
@media only screen and (min-width: 320px) {
	.header-img {
		width: 320px !important;		
	}
}
.repo > .mdl-card__title {
  color: #fff;
  height: 176px;
  background: url('../img/background.jpg') center / cover;
}
.repo > .mdl-card__menu {
  color: #fff;
}

Set min-width as 320px rather than 350px
Set min-width as 320px rather than 350px
