1)  In the main Reaper resources folder, navigate into the ColorThemes folder.
If you don’t know where the resources folder is, you can go in Reaper → Options → Show REAPER resource path (like shown below ↓).

2) Copy all the reARK theme files from the downloaded .zip into the ColorThemes folder.

It’s installed.
Now you can just select it in Reaper from Options → Theme → reARK.