The code you suggested only worked when I put ‘!important’ at the end of each line.
Then, to make the link appear underlined only when the user hovers over it, I tried applying the original CSS to .entry-content a:not(.more-link):hover. However, the whole link text ended up having the blue background instead of just a line underneath (I’m guessing it’s just a bug in the theme).
I finally managed to solve this problem by looking at one of the examples from this discussion thread:
.entry-content a:hover {
color: #2ab2f5;
background: linear-gradient(#fff 95%, #2ab2f5 5%)!important;
}
It now works just the way I wanted.
Thanks again for your help.