Update Epik theme

Hi,
We have a website that has the Epik theme installed for quite a few years. We are trying to add WooCommerce to it and we noticed the theme has gone through some changes since then. Is there a way to update to the new theme without losing custom CSS? There is quite a lot of CSS added.

One of the issues we saw now is that the checkout button doesn’t appear. There is no plugin conflict so thinking the theme was installed before it had woocommerce support?
Thank you.