No need to start counting syllables on your fingers for "Theme in Yellow." There's none of that fancy form and meter stuff going on here. It's all free and conversational-like for our pumpkin-speaker, who prefers to keep things informal and fun.
We know Sandburg really liked writing in this sort of conversational tone, so we often see a lot of enjambment and idiomatic phrasing (common talk) going on in his work. For instance, lines 6-11 are all one thought and image that capture the "last of October" in a way that's simple and recognizable for all readers ("children join hands and circle round me singing ghost songs). So that smooth conversational approach really helps us to imagine the scenery in a way that everyday folks can really dig. And if you need some further proof, just take a look his poems "Fog" and "Chicago". They also sound like "America singing."
So what we get in "Theme in Yellow" is a light-spirited, conversational form that's not anchored to any one pattern or rhythm. The lines are also short and to the point, which fits well with our speaker who's a pumpkin and presumably didn't hit up Yale for a bunch of poetry courses.