Is it required to code for CSS in html code workplace or we can do separately as well or how to link it?

Answer 1

In SL, the three files (HTML, CSS and JS) are implicitly linked to each other. Outside SL though, you need explicit linking. For CSS, use the link tag:
<link rel="stylesheet" href="yourfile.css"/>
If the CSS file is in the same folder as the HTML, specifying the name is enough.

Answer 2

You don't have to clutter your HTML workspace with CSS. Just create a css file within the same directory and use a link tag to link your stylesheet to your HTML.