IPython/frontend/html/notebook/static/codemirror2/lib/codemirror.css

.CodeMirror {
  line-height: 1em;
  font-family: monospace;
}

.CodeMirror-gutter {
  position: absolute; left: 0; top: 0;
  background-color: #f7f7f7;
  border-right: 1px solid #eee;
  min-width: 2em;
  height: 100%;
}
.CodeMirror-gutter-text {
  color: #aaa;
  text-align: right;
  padding: .4em .2em .4em .4em;
}
.CodeMirror-lines {
  padding: 0.4em;
}

.CodeMirror pre {
  -moz-border-radius: 0;
  -webkit-border-radius: 0;
  -o-border-radius: 0;
  border-radius: 0;
  border-width: 0; margin: 0; padding: 0; background: transparent;
  font-family: inherit;
}

.CodeMirror textarea {
  font-family: monospace !important;
}

.CodeMirror-cursor {
  z-index: 10;
  position: absolute;
  visibility: hidden;
  border-left: 1px solid black !important;
}
.CodeMirror-focused .CodeMirror-cursor {
  visibility: visible;
}

span.CodeMirror-selected {
  background: #ccc !important;
  color: HighlightText !important;
}
.CodeMirror-focused span.CodeMirror-selected {
  background: Highlight !important;
}

.CodeMirror-matchingbracket {color: #0f0 !important;}
.CodeMirror-nonmatchingbracket {color: #f22 !important;}