The same risks apply to any software proprietary or open source which is why Microsoft have the following in their licence agreement:

12. DISCLAIMER OF WARRANTY.The software is licensed “as-is.” You bear the risk of using it. Microsoft gives no express warranties, guarantees or conditions. You may have additional consumer rights under your local laws which this agreement cannot change. To the extent permitted under your local laws, Microsoft excludes the implied warranties of merchantability, fitness for a particular purpose and non-infringement. FOR AUSTRALIA ONLY: You have statutory guarantees under the Australian Consumer Law and nothing in these terms is intended to affect those rights.

LIMITATION ON AND EXCLUSION OF REMEDIES AND DAMAGES. You can recover from Microsoft and its suppliers only direct damages up to U.S. $5.00. You can't recover any other damages, including consequential, lost profits, special, indirect or incidental damages.

Knowing that and knowing that themes can have code is two different things though. I wasn't particularly surprised as I thought (maybe wrongly) that global themes just installed all the other bits which would require code.