Per-character diff highlighting not displaying in Emails on Push

Summary

Per-character diff highlighting CSS styles are missing from emails sent by the Emails on Push service.

Steps to reproduce

  1. Enable emails on push service
  2. Create and push a commit that changes some characters in a line
  3. Observe email describing pushed commit

What is the current bug behavior?

HTML markup is inserted into the emails to provide elements to style for inline-diffs, but the appropriate CSS to style those classes/elements is missing.

What is the expected correct behavior?

The HTML elements are styled, allowing diffs in emails from the Emails on Push service to show the characters in a particular line that have changed.

Relevant logs and/or screenshots

Current behaviour:

Screenshot_from_2017-10-31_11-28-29

Expected behaviour:

Screenshot_from_2017-10-31_11-29-13

Output of checks

This bug happens on self-hosted GitLab CE 10.1.0.

Possible fixes

The following styles are present in the GitLab web interface, but not in the emails sent by the Emails on Push service:

.code.white .line_holder .line_content.new span.idiff {
  background-color: #c7f0d2;
}
.code.white .line_holder .line_content.old span.idiff {
  background-color: #fac5cd;
}