I’m afraid this isn’t possible at the moment, but it’s on our roadmap.
There’s a workaround for the meantime: Check out each app with Git, copy the /theme directory from one to the other (or those parts of it you want to be inherited) and push upstream.
Something like
git clone <url for app A>
git clone <url for app B>
cp -R app_A/theme/* app_B/theme
cd app_B
git add theme
git commit -m 'Cloned theme from app_A'
git push origin