There are no buses from Downtown Disney to the theme parks, so I assume the reverse is also true. You could take the boat to the Swan/Dolphin and transfer to a bus to DD, or take a bus to Saratoga Springs and walk to DD.
PPs are correct in that there are no buses between DTD and any of the parks. Guests were parking for free at DTD to avoid parking fees at the parks, leaving no parking for anyone who was visiting DTD so Disney stopped those buses several years ago.
If you take the boat, S/D is the first stop and would make sense as a transfer point. if you're walking, you would have to go past BW to get to S/D to transfer, so I would just stop at BW and catch a bus from there.