Can light agents edit the source code of a theme in Zendesk Guide?
No. Only a Guide Manager has editing rights to make changes to the source code.
If you have a Guide Professional or Enterprise subscription, light agents can add and edit articles and posts in knowledge base sections and community topics where agents have permission. For more information, see the article: Understanding Guide roles and privileges.
For more information about the light agent permissions, see the article: Understanding and setting light agent permissions (Collaboration Add-on).