As a web designer building themes in wordpress should you learn php?

As Roba said, you will need to learn php, of course not becoming an expert or something.
Since wordpress theme/template built on php template engine You will need to learn basics of the language and how to write syntax correctly in order to be able to build the theme.
This is also common for others like Drupal, joomla, moodle, etc..

It's recommended.
I'm a designer who worked with Wordpress in the early stages of my career, I didn't become a pro at it, but having a general sense of how it works definitely helped a lot.