Use Hugo extended for Sass/CSS (#13113)

assets/sass/_reset.sass

@@ -0,0 +1,61 @@
+html, body
+  margin: 0
+  padding: 0
+
+
+input, button
+  outline: none
+
+button
+  cursor: pointer
+
+ul, li
+  list-style: none
+
+ul
+  margin: 0
+  padding: 0
+	
+a
+  text-decoration: none
+
+// CONVENIENCE
+.clear
+  display: block
+  clear: both
+
+.light-text
+  color: white
+
+.right
+  float: right
+
+.left
+  float: left
+
+.center
+  text-align: center
+
+
+//mixins
+@mixin fullScreen
+  position: fixed
+  top: 0
+  left: 0
+  width: 100vw
+  height: 100vh
+
+
+@mixin pureCenter($left: 50%, $top: 50%)
+  position: absolute
+  top: $top
+  left: $left
+  transform: translate(-50%, -50%)
+
+
+@mixin maintain-aspect-ratio ($width-factor: 16, $height-factor: 9, $target-width: 80vw, $target-height: 80vh)
+  width: $target-width
+  height: $target-width * ($height-factor / $width-factor)
+  max-width: $target-height * ($width-factor / $height-factor)
+  max-height: $target-height
+