Hi Yumiko,
The extension outputs the default fields’ HTML with CSS comes from Meta Box plugin. The style is basic and works in most cases. However, to make it match perfectly with your theme’s style, you need to do some work with CSS.
You can also add some code before and after the form by using the form's hook rwmb_profile_before_form
, rwmb_profile_after_form
https://docs.metabox.io/extensions/mb-user-profile/#form-actions