you can estimate this from vibrational spectrum (phonons) or heat capacity (phonopy calculates). here's bit theory https://arxiv.org/pdf/1103.3146.pdf and http://www.phy.pku.edu.cn/fsc//admin/fileadmin/upfile/Pat/1417399725624.pdf
Hi, there complete derivation of the Debye model in the second link I have sent (starting slide 8). if you have energy/frequencies or Cv, then theta can be simply calculated.