Usually themes emerge from the connection between two or more codes. But is it possible to have a theme that is based on a single code? Is there a reference supporting that?
Yes, a theme can emerge from a single code in a thematic analysis, although this is less common than themes being formed from multiple codes. Such code is particularly "meaty" or complex, may contain multiple facets of a singular concept.
Let me add some context to an answer. Yes. No. It depends.
Where is the code? Descriptive, after seeing a phrase or comment? Interpretative, after forming subcategories, categories, and larger categories? Location, location, location. Also, inferences matter; Brandom speaks about the power of inferential control, or how our minds often write conclusions from our expectations.
What is the coding trail? That is, just because a code exist does not mean every code tells the same story and means the same thing. I recommend everyone add subcodes to codes--even after initial formation--to account for nuances. A common example I present to my students in seminars is the many meanings of happiness; I give an example of a roller coaster, graduating from college, and having a healthy baby. All three can be labeled happiness, but the kind of happiness all tell a very different story.
I am skeptical. A single code is a label that reduces a participant to a single word or small phrase. Expand the narrative.