环境是 rails 4.1 对着书敲代码 第 5 章的时候报错

Sass::SyntaxError in StaticPages#home
Showing /home/david/begin/sample_app/app/views/layouts/application.html.erb where line #5 raised:

Undefined variable: "$grayLight".
  (in /home/david/begin/sample_app/app/assets/stylesheets/custom.css.scss:58)

其中 custom.css.scss 代码如下

@import "bootstrap";

//= depend_on_asset "bootstrap/glyphicons-halflings-regular.eot"
//= depend_on_asset "bootstrap/glyphicons-halflings-regular.svg"
//= depend_on_asset "bootstrap/glyphicons-halflings-regular.ttf"
//= depend_on_asset "bootstrap/glyphicons-halflings-regular.woff"

$grayMediumLight: #eaeaea;


/* universal */

html {
  overflow-y: scroll;
}

body {
  padding-top: 60px;
}

section {
  overflow: auto;
}

textarea {
  resize: vertical;
}

.center {
  text-align: center;
  h1 {
    margin-bottom: 10px;
  }
}

/* typography */

h1, h2, h3, h4, h5, h6 {
  line-height: 1;
}

h1 {
  font-size: 3em;
  letter-spacing: -2px;
  margin-bottom: 30px;
  text-align: center;
}

h2 {
  font-size: 1.2em;
  letter-spacing: -1px;
  margin-bottom: 30px;
  text-align: center;
  font-weight: normal;
  color: $grayLight;
}

p {
  font-size: 1.1em;
  line-height: 1.7em;
}


/* header */

#logo {
  float: left;
  margin-right: 10px;
  font-size: 1.7em;
  color: white;
  text-transform: uppercase;
  letter-spacing: -1px;
  padding-top: 9px;
  font-weight: bold;
  line-height: 1;
  &:hover {
    color: white;
    text-decoration: none;
  }
}

/* footer */

footer {
  margin-top: 45px;
  padding-top: 5px;
  border-top: 1px solid $grayMediumLight;
  color: $grayLight;
  a {
    color: $gray;
    &:hover {
      color: $grayDarker;
    }
  }
  small {
    float: left;
  }
  ul {
    float: right;
    list-style: none;
    li {
      float: left;
      margin-left: 10px;
    }
  }
}