The theory is quite complex.

Consisting of many interconnected parts or elements, making it difficult to understand or analyze.
Her explanation was too complex for me.
The project has many complex requirements.
Understanding the rules can be complex.
complex problem
complex system
complex issue
complicated
intricate
difficult
The puzzle was too complex for me to solve.

Consisting of many different and connected parts.
Her explanation was complex and hard to follow.
The project has a complex design.
Understanding the rules can be complex.
complex problem
complex system
complex relationship
complicated
intricate
difficult
Don't complex the issue with too many details.

To make something more complicated or difficult.
Please don't complex the plan with extra steps.
She tends to complex her explanations unnecessarily.
Let's not complex the discussion with unrelated topics.
complex the situation
complex the process
complex the conversation
complicate
intricate
confound
"The complex word made the text hard to read."

Describing something that is complicated or difficult to understand.
The complex puzzle took hours to solve.
Her complex explanation confused everyone.
The complex rules made the game hard to follow.
complex problem
complex idea
complex system
complicated
intricate
difficult