Single-step form
You can add the following codes into the custom CSS box of the Design tab of your editor:
.formaloo__button {text-align: center; margin:auto;}
.formaloo__submit-button {text-align: center; margin:auto;}
.formaloo__navigation-button {text-align: center; margin:auto;}
Multi-step form
You can paste the following code into the custom CSS box on the Design tab of your editor:
div[class^='form-components-style_submit-wrapper'] {display: flex; flex: auto; justify-content: center;}