What's the CSS or any file that we can use to change styles?

You can find it through a tool like Firebug, but you shouldn’t change it. Use a plugin to add your CSS modifications: https://www.ads-software.com/plugins/custom-css-manager-plugin/

But the same kind of CSS comes with Virtue theme also, right? To do some changes to my theme (like changing hover colors & size of icons) I don’t know the classes, property names that are used in CSS. So I need to analyse CSS first. Firebug cannot help on this case, can it? How can I know the names that are used in virtue theme CSS to declare each item on theme?

Yes Firebug can help you analyse this, it exposes the entire stylesheet too so it’s not just little bits of CSS too. See https://getfirebug.com/css.php

It’s working, Thanks!