CodeMirror has just landed in WordPress 4.9-alpha: https://core.trac.wordpress.org/changeset/41376
The Custom CSS extensions in Jetpack should be updated to build on top of the new code editor APIs rather than include separate copies.
For previous Custom CSS work which moved the feature into the Customizer with WordPress 4.7, see #5539.
/cc @georgestephanis