You can actually use NPM to install from a specific branch of a GitHub repo. Just do npm install NodeBB/nodebb-theme-persona#timeline-style
Right now the version in package.js on this branch is the same as on the main branch (10.3.16) so it should work without any changes to NodeBB package.json (not sure if the version of installed theme is checked at the startup, but with the same version it doesn't matter)
Edit: but as Julian said it's work in progress and, so it may be buggy. Also, I'm not sure if NPM will update it from its registry automatically on your next nodebb upgrade or if you'll have to manually install it back.
