How do I cancel the highlight around a journal item in Outlook

I hope this question is clear! I keep a journal in Outlook, categorising my
different kinds of content (i.e. my diary, food diary, exercise diary,
etc)under 'types', and every now and again you find an entry has a highlight,
which is perpetuated for ever down the years. This creates a division
between the items and I would be grateful to know how I accidentally created
this highlight, what it is for, and how to delete it.

Thank you.