nodebb mobules should either start with nodebb-plugin- or nodebb-theme- so in your case your theme name should be nodebb-theme-exo-theme

I had the same problem when I was playing with NodeBB on a $2.5 vps from Vultr (=512MB ram). htop shows me that build process uses all available ram and cpu. Deploying on a decent machine/vps would solve the problem, aternatively, I called build steps one at a time ( eg ./nodebb build languages , ./nodebb build 'client side styles' etc) . Hope this helps.