Do we have any modern art fans out there? We can hear you causing a ruckus, probably because you're digging this poem's title so much, "Theme in Yellow." So what's the connection between this poem and modern art, besides the period? Well, for one thing, artists and writers were really into the idea of having "themes" in their works, which is just a fancier way of saying they had a bigger picture in mind before putting ink to paper, or paint to canvas. What was actually on the canvas was not the main thing; the idea behind the shapes and colors was what mattered.
There's definitely some of that visual art-inspired approach going on in Sandburg's poem, since he opens with a focus on color and transforms those "yellow balls" into "orange clusters" later on. In other words, he's really working with the different shades of yellow in this particular poem and how they help to define autumn and Halloween. So we have a definite sense of a theme happening here, both in terms of colors and the time of year they represent.