You can determine which tiddler content types are edited by the ~CodeMirror widget by creating or modifying special tiddlers whose prefix is comprised of the string `$:/config/EditorTypeMappings/` concatenated with the content type. The text of that tiddler gives the editor type to be used (eg, ''text'', ''bitmap'', ''codemirror'').
The current editor type mappings are shown in [[$:/ControlPanel]] under the "Advanced" tab.
! ~CodeMirror Configuration
You can configure the ~CodeMirror plugin by creating a tiddler called [[$:/config/CodeMirror]] containing a JSON configuration object. The configuration tiddler must have its type field set to `application/json` to take effect.
See http://codemirror.net/ for details of available configuration options.