But for 64bit we don't have this but still, we can have themes and can those are to be selected manually.
The most complete 64bit GIMP (v. 2.8.10) I have is here: link.<-This one
I hope the link works since that GIMP is already themed and has some useful extra plugins included.
To add themes to your own GIMP install (64bit) you will need to place some files in a few folders:
..\GIMP-2.8\lib\gtk-2.0\2.10.0\engines (here you add the theme engines: dll's found in previous links)
..\GIMP-2.8\share\themes (here you need the theme settings files, get them from here or the first two links)
Regarding the already-themed GIMP:
You can extract the zip and use the program. Placing it in Program Files means the same as installing (except you don't have context menu option only if you install the program first and then replace all files with the ones provided here.