added ExtensionRoot for consistent styling across injected components, and added a bunch of comments for all the added types and classes

src/views/components/injected/CoursePopup/CoursePopup.tsx

@@ -1,16 +1,19 @@
 import React from 'react';
 import { Course } from 'src/shared/types/Course';
+import { Button } from '../../common/Button/Button';
 import styles from './CoursePopup.module.scss';
 
 interface Props {
     course: Course;
+    onClose: () => void;
 }
 
-export default function CoursePopup({ course }: Props) {
+export default function CoursePopup({ course, onClose }: Props) {
     return (
         <div>
             <h1>{course.fullName}</h1>
             <p>{course.description}</p>
+            <Button onClick={onClose}>Close</Button>
         </div>
     );
 }