Here's a primer to get OneStepCheckout styling started on Ultimo Magento 2 theme

You can place this isnide OneStepCheckout css or create a separate script and address OneStepCheckout layout handler checkout_onestepcheckout_iosc or just override the OneStepCheckout default css file in theme context

More information on Magento 2 frontend development can be found on Mgento devdocs areas: https://devdocs.magento.com/guides/v2.2/frontend-dev-guide/bk-frontend-dev-guide.html