How can I customize the look of a form generated by PaidMembershipPro plugin?

I am new to using WordPress but would like to create a website for a small organization. I installed the PaidMembershipPro plugin along with the "WP Bootstrap Starter" theme. I also used the PaidMembershipPro plugin settings tab to create the required paged.

However, the forms and the pages don’t look good. I would like to use bootstrap classes to make it look better. So I tried editing the Membership Levels page, but all I see is [pmpro_levels]

How can I edit the HTML code generated by this [pmpro_levels] code?