Hello Kent,

I hope you are doing well!

To changes or add the CSS for Log Note created by Log Note button inside chatter you can use selector .o-mail-Message[aria-label="Note"].

I've attached screenshot where I've added red border for log note for your reference.

I Hope this information proves helpful to you.

Thanks & Regards,

Email:  odoo@aktivsoftware.com           

Skype: kalpeshmaheshwari