You just need to add a function to style the login form:
function my_login_css() {
?>
<style type="text/css">
#wp-submit {
border-color: #777;
background: #999;
box-shadow: none;
text-shadow: none;
border-radius: 0px;
}
#wp-submit:hover {background: #ccc}
</style>
<?php
}
add_action('login_enqueue_scripts', 'my_login_css');
When using a theme, like Avada, it’s best to add these custom functions to a child theme. More information, specific to Avada.