lib/codemirror-3.1/addon/hint/simple-hint.css

.CodeMirror-completions {
  position: absolute;
  z-index: 10;
  overflow: hidden;
  -webkit-box-shadow: 2px 3px 5px rgba(0,0,0,.2);
  -moz-box-shadow: 2px 3px 5px rgba(0,0,0,.2);
  box-shadow: 2px 3px 5px rgba(0,0,0,.2);
}
.CodeMirror-completions select {
  background: #fafafa;
  outline: none;
  border: none;
  padding: 0;
  margin: 0;
  font-family: monospace;
}