Improve colours for an Inline Comment's highlighted text in the notification email for dark mode clients

Issue Summary

When receiving an email notification for an inline comment, the highlighted text (the part the comment refers to) is hard to read if using a dark-mode enabled email client.

Steps to Reproduce

1. Have a dark mode enabled email client (i.e. Mac Mail and set MacOS appearance to dark)
2. Create a page, add some text
3. With another account, add an inline comment to some text
4. Wait for the notification email
5. Notice the highlighted text is very hard to read

Expected Results

The highlighted text should be easily readable

Actual Results

Highlighted text is barely readable

Workaround

Not a real workaround but adding regardless: Switch email client/OS to light mode