You’re forgiven ?? You don’t learn what you need to know unless you ask questions. Na?veté in questions is expected. The purpose of themes are to manage the appearance and presentation of your site. The purpose of plugins is to extend your site’s functionality. Ideally the two are mutually exclusive and any theme should work with any plugin and vice versa. In practice that’s not always the case, especially with commercial products. It’s generally true of plugins and themes found in the WP repository here.
Getting your WP site to match the appearance of your current site could prove to be difficult. The content can be identical, but unless the theme is custom developed, appearance will likely be different. This is a good opportunity to freshen your site’s appearance.
There are thousands of plugins that do all sorts of things, yet new, unique needs come up every day where there’s no ideal existing plugin and some custom development is needed to achieve certain functionality.