Bootstrap is perhaps the most widely used front-end framework on the web. It was released in August, 2011, and has been under continuous development ever since.
Skilling comes with a custom theme based on Bootstrap 4. Although the theme could be used for any Drupal site, it is designed for students taking skills courses. For example, students doing tasks will want to split their screen space. One half will have a browser showing a Skilling course. The other half of the screen will have and a worksheet, code editor, or whatever students use.
Skilling's theme will help. It makes best use of available screen space. All pages stretch horizontally to use the entire browser window. Students can collapse sidebars at will. On a large monitor, Skilling will pretend it is on a mobile device, if students make its window small enough.
Here are screenshots of one lesson. (It's on a test site, so the content makes no sense.) First, here's the lesson as it might be on a laptop, with a largish browser window.
The main menu is fully visible, and there are sidebars on the left and right.
Suppose a student wants more screen space to, say, run Excel, and still see the lesson. S/he hides the sidebar, and shrinks the browser window by a third.
The student decides Excel needs even more space. S/he shrinks the browser window down to mobile-phone size.
The main menu is hidden behind a hamburger icon (the three lines in the upper right). The video has shrunk.