We can go very deep into theme from Shaw's work of literature by talking about idealism and realism and how they exist in regards to both war and love, but the truth of the matter is that one can sum up the themes of Arms and the Man by focusing on the theme of appearance vs. reality. Let's look at this theme in regards to war, society, and love.
In regards to war, the "appearance" of war is one of idealism. Soldiers gloriously fighting in battle and coming home heroes. One need only to look at Raina and Sergius to see war in how idealism is displayed. "The dream of patriots and heroes!" The reality of war, of course, is displayed more in the character of Bluntschli. I have always found it ironic how the word "blunt" is found right in his name. Bluntschli insists that war has a negative effect on the men participating. They want food and sleep. Basically, they want to be anywhere but on the battlefield. Very far from the idealistic appearance of war as a glorified hero. Shaw might say that Raina and Sergius learn the truth through the course of the work.
In regards to society, although the appearance of class and distinction seem positive, the reality is distinctly different. For example, even though Bluntschli has a fairly low rank, his intelligence far outweighs that of his superior officers in many ways. In regards to society as a whole, one need only look at the flippant Catherine to see Shaw's commentary on class distinction. Catherine, of course still hold the "appearance" of high society.
Finally, love can also fall into the appearance vs. reality theme. It is NOT a surprise that Shaw approaches this subject yet again. (He is a big fan of approaching it in his other works of literature.) It is the relationship between Raina and Sergius that best exemplified the appearance vs. reality theme in regards to love. Raina has been instructed to love someone like Sergius. Sergius has been instructed to seek out someone of Raina's social status. Both of them are fooling themselves in choosing each other, each pretending to be something they are not. Only by the end of the work do they discover their true selves. Idealism has lost out yet again.