don’t use a theme, get a plugin such as https://wpjobmanager.com/ and then add a theme on the top.
if you buy a theme with the built in functionality, you won’t be able to extend it, WP Job Manager is regularly updated and has plenty of extensions, or you can build custom ones if you need a specific function.
The find a theme that you like and integrate the plugin into it, you then get a design you can change whenever you want and a separate functionality plugin which receives updates.