Underlining on “Visitors per day” of the “Count per day” plugin

This number has an hint/title on mouse over.
Google HTML tag <abbr>.

OK. How can I eliminate the underlining?

I found in Google that I have to add
abbr[title], acronym[title] {
border-bottom: none;
}
In which plugin file do I need to to do the addition?

It has been resolved, I added in Simple Custom CSS
abbr {text-decoration:none;}