Hello shaka,
you can add this CSS on the page level and if you want in whole application then on the application level

.app-menu-links a:hover {
    color: #55f1bc;
    text-decoration: none;
    background-color:#fff;
}

Color and background color you will add as per your requirement

note( this in the menu from OutSystems and in the menu its linked to page (top or side menus doesn't matter)if you add similar approach(by default) then this will work )

for another think how to find out and fix the CSS already mention in this post
https://www.outsystems.com/forums/discussion/70419/change-hover-link-color/

hope this will helps you,

Thanks and Regards,
Akshay Deshpande