<div class="wrapper">
  <h1>Simple css Alerts</h1>
  <div class="alert">Hi! I'm simple alert! Check how I look :)</div>
  <div class="alert alert--primary">Hi! I'm simple alert! Check how I look :)</div>
  <div class="alert alert--warning">Hi! I'm simple alert! Check how I look :)</div>
  <div class="alert alert--danger">Hi! I'm simple alert! Check how I look :)</div>
  <div class="alert alert--success">Hi! I'm simple alert! Check how I look :)</div>
</div>

$primary: #3498db;
$default: #34495e;
$warning: #e67e22;
$danger: #e74c3c;
$success: #2ecc71;

@mixin alertCreator($color) {
  padding: 15px 30px;
  margin-bottom: 15px;
  border-radius: 3px;
  border-bottom: 3px solid darken($color, 10%);
  letter-spacing: 0.5px;
  color: rgba(#fff, .8);
  background-image: linear-gradient(lighten($color, 15%), $color);
  box-shadow: 0 5px 10px rgba(#000, .3);
}

* {
  box-sizing: border-box;
}

/* 
** just for example
*/

h1 {
  margin-bottom: 2rem;
  text-transform: uppercase;
  color: $default;
}

.wrapper {
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  min-height: 100vh;
  padding: 30px;
}

/*
** Alert styles
** also we can use @each or some one else for generating this alert, but this solution looks simpler :) 
*/
.alert {
  @include alertCreator($default);
}

.alert--primary {
  @include alertCreator($primary);
}

.alert--warning {
  @include alertCreator($warning);
}

.alert--danger {
  @include alertCreator($danger);
}

.alert--success {
  @include alertCreator($success);
}