Answer 1

Add inside head using link tag like
<link rel = "stylesheet" href = "file1.css">
<link rel = "stylesheet" href = "file2.css">

Answer 2

💞𝐒𝐨𝐦𝐲𝐚💞 This is live application which doesn't provide us to read and write file so we can't do here.
If you want to apply CSS on different case then you have to write JS code. We can apply CSS using Javascript or JQuery also. For example
$("#abc").css("color", "red");
or
document.getElementById("abc").style.color = "red"

Answer 3

💞𝐒𝐨𝐦𝐲𝐚💞 Your file should be external. On Sololearn you can write all CSS inside CSS tab only.