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If you have the exact versions of the mods you installed before (should be under folder / Packages on server) then you can use those and then copy over the Theme and it should work in principle - HOWEVER - that has a lot to do with how the current database was installed and the files used because some mods may not want to install on a new version of SMF - you will have to test to ensure it works.
If you are using the old database then that is not an issue. You only have to copy over the modifications.
I once had to copy a DB from one domain to another that I sold and it was a real pain. I had to change the urls in the DB that pointed to the old site then the skins and source files, I think it took me all day to do it.