Add hover underline to two links in Wordpress

Hi there, I have a website which is on the wordpress platform. At the bottom of the page you can cycle through posts by click on "newer page" and "older page" links. I want these links to underline when hovered but I haven't been able to figure it out myself. Your job will be to not only make them underline on hover but also tell me where you had to edit the code to make this occur. Thanks!