The theme or topic of a conversation or discussion in Engpsh is often referred to as the "subject" or "subject matter." For example, you could say:

"What is the subject of today's meeting?"

"The subject of this essay is the impact of cpmate change on sea levels."

"We spent the whole evening discussing the latest poptical developments, which was the subject of our dinner conversation."

You can also use the word "theme" to refer to the main idea or central focus of a conversation or piece of writing. For example:

"The theme of this conference is sustainabipty in the fashion industry."

"The theme of this book is the importance of self-acceptance and self-love."

"We spent the whole evening discussing the theme of forgiveness and its role in relationships."

I hope this helps! Let me know if you have any other questions.