Apply Twitter Bootstrap stylings to CodeMirror?

Code Mirror and Bootstrap (v3 and v4):
Styling does not yet support validation states (has-error, has-warning, has-success)

.CodeMirror {
  /* Bootstrap Settings */
  box-sizing: border-box;
  margin: 0;
  font: inherit;
  overflow: auto;
  font-family: inherit;
  display: block;
  width: 100%;
  padding: 6px 12px;
  font-size: 14px;
  line-height: 1.42857143;
  color: #555;
  background-color: #fff;
  background-image: none;
  border: 1px solid #ccc;
  border-radius: 4px;
  box-shadow: inset 0 1px 1px rgba(0, 0, 0, .075); /* Bootstrap 3 */
  box-shadow: none; /* Bootstrap 4 */
  transition: border-color .15s ease-in-out, box-shadow .15s ease-in-out;
  /* Code Mirror Settings */
  font-family: monospace;
  position: relative;
  overflow: hidden;
}
    
.CodeMirror-focused {
  /* Bootstrap Settings */
  border-color: #66afe9;
  outline: 0;
  box-shadow: inset 0 1px 1px rgba(0, 0, 0, .075), 0 0 8px rgba(102, 175, 233, .6); /* Bootstrap 3 */
  box-shadow: 0 0 0 .2rem rgba(0, 123, 255, .25); /* Bootstrap 4 */
  transition: border-color ease-in-out .15s, box-shadow ease-in-out .15s;
}

CodeMirror hides the original textarea and creates a (fairly complex) structure of div and pre elements. You can style the outermost div which has a class of .CodeMirror to achieve the same effect.

This will require customizing the CodeMirror stylesheet or adding your own style for the class/element. If you are building Bootstrap using LESS, there may be a way to apply a mixin to avoid duplicating the textarea style, though the amount of duplication is probably minimal.