All comics have themes.
In order to have a successful comic you need to have a theme. It creates boundaries that you have to work within, which in turn allows you to be more creative and compelling. Garfield is about a fat cat and his owner, with other animal characters that communicate through thought bubbles. Calvin and Hobbes is about a young boy and his imaginary friend/stuffed animal Hobbes and their adventures together. Beetle Bailey is about a lazy soldier in the army and his strict and easily angered sergeant. When you break them down, these are all rather unusual story ideas to start with. Not everyone is going to be able to relate to being in the army, or owning a cat. But within these narrow themes, larger ideas and thoughts can be communicated. Calvin and Hobbes is known for being a startlingly insightful comic, and every reader can relate to the innocence and wonder shown in the comic. The characters in these comics echo sentiments we ourselves feel or can relate to in some way.
These two newspaper comics have both taken the approach of drawing/writing about family life, with all of its trials and tribulations. However, they took two very different approaches.
For Better or For Worse has characters that have grown throughout the years. It started in 1979 and had a family with babies, and then had those babies grow up and have kids of their own. After 29 years of continuous strips, the author retired from the daily production of strips. But because of how popular the comic strip and its characters were, all of the comics are being republished, through the newspaper format. There are a few minor changes or new strips, but for the most part fans are getting the opportunity to grow up with the characters all over again.
Zits on the other hand has a theme based on a high school teenager living with his parents. Because the theme is specifically about teenagers, the characters in the strip don’t age. The look of the comic sometimes does, and the content changes a bit to accommodate changes in the world (smartphones and other technology, as well as fad changes) or to advance the characters (his friend Hector growing facial hair, him dating Sara, etc.). But the content and theme of the strip don’t change, and so the comics remain fairly consistent and easy to pick up on any given day.
Webcomics function much the same way. They have a set theme that they start out with, whether that’s corrupted versions of care bears, evil scientists, superheroes, or college. But the major difference for webcomics is that most start with a specific story in mind for that theme. There’s a lot of variation within that, but because of the nature of webcomics, many authors are taking advantage of the format to create graphic novels online.
In this format, updates are published in the form of one page of a comic, rather than a self-contained joke. This can be both excellent and frustrating, as it is dependent on a consistent readerbase and regular updates. But it’s a very effective form of creating a graphic novel, because it establishes a fanbase before your comic is even published.
Ultimately, themes are the box within which you build your comic. Sometimes they expand beyond where you ever expected them to go, and others exactly follow the original plan of the theme and storyline. It’s impossible to predict where your story will take you.