As you create Html file same like as css file. Save it with .css extension. As you know there is 3 types to apply css in Html.
1-using inline CSS
2-using internal CSS
When you want to apply too much inline CSS but same style in every Html file then you need to create external file.
So there is no need to write same CSS again and again in multiple html pages
Better to write in single file and import in multiple html pages.
This feature may not be available on solo learn. But it works on mobile as well as in computer. I do not know how to link external css in solo learn. There is a css style sheet in solo learn but that doesn't need link tag. Make sure all your files is in same folder or you know paths of different your css files. Write path of your file in place of my style. css
It should work on mobile phones also. But in solo learn. It may not work.
This code is for changing background. It has external css but it doesn't need link tag. As this code is in solo learn but out side of solo learn this css will not work if do not use link tag in your html
This is used when you apply external CSS to your page. You create a new file with the extension ".css" and link it to your html part.
The <link rel="stylesheet" from my understanding, I think means that you're making reference to a stylesheet page.
The attribute type="text/css" is there to explain that the stylesheet page you're making reference to is in text
The last part (href="mystyle.css") is the path of the file to be used.