Someone may have an easier solution, but with Bootstrap 4 you need to edit the CSS like the following code. (I have edited it, so the hamburger will display white on your webpage. We can show you how to modify it if you want to change colours.
<style>
.navbar-toggler-icon {
background-image: url("data:image/svg+xml;charset=utf8,%3Csvg viewBox='0 0 32 32' xmlns='http://www.w3.org/2000/svg'%3E%3Cpath stroke='rgba(255,255, 255, 1)' stroke-width='2' stroke-linecap='round' stroke-miterlimit='10' d='M4 8h24M4 16h24M4 24h24'/%3E%3C/svg%3E")!important;
}
</style>You can add this to the header section of the page.
You can add the class “navbar-toggle-icon” to the class manager, but you won’t be able to change the colour of the burger, just the background and border etc.