Install reARK
Installing reARK is a manual theme drop. No ReaPack required.
Three steps
1 / Locate
In Reaper, open Options and click "Show REAPER resource path", then open the ColorThemes folder.
2 / Copy
Copy all the reARK theme files from the downloaded .zip into ColorThemes.
3 / Activate
Select it from Options, Themes, reARK.
Options, Show REAPER resource path