Hi Judy,
You can use the following custom css to remove the border-radius from the navigation bar.
.main-navigation li ul {
border-radius: 0px !important;
}
and to change the font and font size of the primary navigation you can use the following custom css
ul.nav-menu li a {
font-size: 2rem;
}
ul.nav-menu > li > ul.sub-menu > li > a {
font-size: 2rem;
}