Theme is the meaning or idea that the artist wishes to impart to the viewer.
This will be achieved through the use of visual clues and relationships, which may or may not be obvious unless the viewer understands the clues.
This painting Good Day, Monsieur Courbet depicts a chance meeting of the painter, his patron Alfred Bruyas and Bruyas’s servant Calas, on a road outside Ornans.
At first glance we notice the painter depicting himself as a man of the road, a rucksack on his back, meeting Bruyas with a "hail fellow well-met" appearance.
Courbet, who welcomed Bruyas as a kindred spirit who would help him "live on my art for all of my life, without ever departing an inch from my principles, without ever for an instant lying to my conscience ... I am right! I am right! I have met you. It was inevitable because it is not we who have encountered each other but our solutions."
However, under normal circumstances it might be expected that the artist would show deference to his patron, on whom he depended for his livelihood.
At the very least, you might expect that the two would be depicted as equals.
However, note the way in which only Courbet stands solidy on the earth, feet apart, with his head tilted back slightly and his beard tipped forwards. He is the one standing directly in unfiltered light. His walking staff is also longer and thicker than that of his patron's walking stick.
Also note the way in which Bruyas has removed his glove, presumably in order to shake the hand of the artist - he is demonstrating an open and friendly posture, but appears to be 'looking up to' the artist.Note also the long shadow cast by Courbet - yet neither the patron nor his servant cast any shadow at all.