You need to find the CSS class for each type of button and then change the shape to circular using a CSS code inserted to Admin Settings> Theme> Advanced Options> CSS> Save.
You can try these as a few examples:
button.ui.primary.right.floated.button.content-input-submit {border-radius:60px; width: 40px; height:40px;}
The (button.ui.primary.right.floated.button.content-input-submit) is the class adress for the "post" button. And the code inside the { } makes the button rounded. You need to play around with the numbers to get it to the correct shape you want.
