I just want to change the theme of my wordpress ecommerce site. because the old developer who built this site used a lot of custom code and 45+ plugins. and as a result, the site speed is very slow (I am using a VPS server). and when I am trying to optimize the speed the custom js and css break the website "note: I follow the all steps according to the documentation". So, my objective is i want to deactivate the all custom js, css code and unusual plugins. On over i just want to change my theme which will help me to fulfill my all the needs. So please help me how to change the theme layout without losings the orders and content. and without affecting the customer order interruption .