If you use CSS Frameworks like Bootstrap or Foundation, the menu class of the menu wrapper element is already in use by the frameworks.
I think whatever class you need, you can manually add it through the snippet: e.g:
{{ craft.olivemenus.getMenuHTML('mainMenu', { 'menu-id': 'main-menu', 'menu-class': 'menu menu-centered' }) }}
If you use CSS Frameworks like Bootstrap or Foundation, the menu class of the menu wrapper element is already in use by the frameworks. I think whatever class you need, you can manually add it through the snippet: e.g:
{{ craft.olivemenus.getMenuHTML('mainMenu', { 'menu-id': 'main-menu', 'menu-class': 'menu menu-centered' }) }}