The Pure CSS page.

Pure CSS is a toolbox of html, css, and js

Pure CSS makes the web app look and feel like a real, native, downloaded onto your phone or device app. Except that this PWA is much, much smaller.

To use Pure CSS with this PureDogPWA template, you'll want to being customizing it to become your own. The HTML and css side-menu are ones you'll likely wish to change. There are many more things that can be done with Pure CSS, see PureCSS.io

You can alter the HTML as you wish. The CSS in side-menu.css can be changed to alter things like the nav bar color. The JavaScript in ui.js. Note that the JS file uses vanilla JavaScript to simply toggle an active class that makes the menu responsive. (You probably don't want to change that.)

Try Resizing your Browser

If you haven't done so yet, try resizing your browser. Watch how things like the navigation controls and images change. That's the "responsive" part of Pure CSS which makes it neat and useful.