Well each theme is different in what can be customized on it. If you want to customize the plan, outside of the featured options in the Customizer tab, then yes, you would need to use CSS. The premium plan is required for that.
I understand that the premium plan gives me access to CSS customization. I could buy the plan, make the changes I want with CSS. My question was what happens if I stop paying for the plan. The customization goes away, presumably, in which case I’d need to keep paying for the plan year after year. Is that the case or is there a “lump sum” option that would allow me to perform customization once and for all?