Document Design states on page 38:

But, as designers, our feelings about what looks "good" may not match the users' feelings. Rather than relying on feelings, a master designer should be able to explain what works and why. Theory is an attempt to provide those explanations—ultimately, to discover and convey fundamental principles of the human experience that can be applied to new design situations.

This is important; it means that design must have transmittable practices and principles to be intelligible and not the result of random or unknowable acts of genius. Theories give us such frameworks. They help us understand how documents function so that we can analyze and construct them competently.

The book offers three theoretical categories:

• Visual perception theories provide biological and psychological explanations of how humans experience the world through their senses—particularly vision. The book provides four subcategories under this head:
• Neurophysiology examines how organisms respond to sensory data through their nervous systems.
• Gestalt theory operates at a broader scale than neurophysiological explanations by asserting that humans don't just see elements such as colors and outlines but whole objects that connect with concepts.
• Constructivism examines how human minds create events and situations through the assembly fragmentary sensory data.
• Ecological perception posits that humans aren't separate, autonomous observers of the world. They know things through their relationships to ourselves and each other.

• Visual culture theories examine how we ascribe meaning to things beyond our pure biological sensing of them. The book parses this topic into three parts
• Visual language asserts that visual marks and symbols (such as a stop sign) accrue conventional cultural meanings.
• Intercultural communication examines how visual elements communicate various meaning in different cultural contexts.
• Semiotics inquires into signs, or the connection between a signifier (the visual icon) and the signified (the associated concept).

• Visual rhetoric is the application of visual perception and visual culture theories toward particular ends. 
• Persuasion, directly influencing an audience to take a particular action or hold a specific position, is one aspect of visual rhetoric.
• User-centered design operates differently than one-way persuasion by acknowledging that agents (designers, clients, users, etc.) all have legitimate needs and agendas that should be balanced for the most effective outcome.