Standards
From MoodleDocs
Styling standard page elements and parts
Moodle developers come from many different backgrounds with different levels of experience. In the past we have tried to define standards for styling components, but this has been implemented inconsistently.
As of Moodle 3.2 the recommendation is for all developers to learn how to apply the classes and components from Bootstrap in order to create the page layouts and components they want to achieve. Custom HTML and styles are strongly discouraged.