What is the name of Liferay Portal's custom Bootstrap theme used in the Classic Theme?

The correct answer is Atlas. Atlas is Liferay Portal's custom Bootstrap theme specifically designed for use in the Classic Theme. It serves as a starting point for developers, incorporating Liferay's design language and principles to create a consistent and user-friendly interface. By using Atlas, developers can leverage Bootstrap's grid system and components while adhering to Liferay's styling, ensuring a seamless integration with the overall Liferay ecosystem.

Other themes or options may not be specifically tied to the Classic Theme in the same way. For example, Lexicon refers to Liferay’s design guidelines but isn't a theme. Base may represent foundational styles but does not denote a unique Bootstrap theme. Bootstrap Classic is not an established name within Liferay’s framework. This highlights the importance of recognizing Atlas not just as a theme, but as a crucial component of Liferay’s approach to theming and design.