Underline Links in Body Text Only

Site URL: https://www.petenginemarketing.com

I have tried a handful of different codes I've found online but they haven't worked in the right way. How can I underline all hyperlinks inside the body text blocks, but not on buttons, navigation, summary blocks, or header 1 or 2 or 3? The code that I have tried has always made changes site-wide. 

Add to Home > Design  > Custom CSS

.html-block a {
    border-bottom: 1px solid red;
}