The code you said you tried, a {text-decoration:none;}, should work. Using, a {text-decoration:none!important;}, should definitely work or using an inline style, like in the <p> tag would take precedence over the stylesheet.
The more correct way would be to either edit the CSS, ideally in a child theme, and apply your own styling. Using !important in the Theme Customizer Custom CSS should be good enough for a mathematician 😉 or anyone else who is doing this as a hobby.