Can't change the radius for buttons…

I was able to resolve the issue with the corners but I still can’t seem to make the three buttons the same size as each other. I also want to make the buttons stack on top of each other when the screen gets too small, rather than overlap on the same row. Here is what my CSS looks like now:

div#header_main {
background-color: white;
}

nav.main_menu {
left: 40%;
}

#top #header .av-main-nav > li > a {
font-size: 18px!important;
}

#top .main_menu .menu li ul a { font-size: 16px; }

body {
font-size: 18px;
}

.widget {
font-size: 16px;
}

.widget {
padding: 1px 1px 0px 1px;
}

body div .avia-button {
border-radius: 5px; }

.avia-button.avia-size-large { font-size: 20px; } /* for large buttons */

@media only screen and (max-width: 767px) {

.avia-button.avia-size-large {
min-width: 0px!important;
}

Thank you