This theme follows Marcus and Juilius around Rome in the time of the ancient Romans (from at least 80CE). The strips frequently feature jokes and puns written in Latin, often requring assistance from the members of the Irregular Webcomic! Forums, and the final translations are rarely, if ever, agreed by all involved in the translation to be correct.
This theme also tends to use more ancient iconography when it interacts with other strips - when Marcus and Julius travel in the afterlife, they meet Charon and Cerberus, which no character previously had done, aswell as it being named Hades (although as this was all revealed to be a hallucination, we can't be sure if this is canonical, but as the characters from the Death theme seem aware of what is going on, it is likely that the afterlife of IWC is simply more complex than previously thought).
Marcus: A member of the Roman senate
Julius: An aspiring member of the Roman senate