Thanks so much!
Few style changes: If you only want to do few style changes, I suggest you to use Custom Css box in Theme Options > General > Display > Custom Css. (this will override all other styles and it saves in database so it stays even if you update the theme)
Heavy style modifications: If you want to do heavy style modifications, I suggest you to find custom-sample.css file in the THEME, rename this file to custom.css and enter styles inside it. Why this way? Because, this file is loaded in the end so the styles you put here will override the previous styles. Also custom.css file doesnt come with theme so this file stays even if you update the theme.
Heavy customization: In case you are a developer and want to do heavy customization, I suggest you use blank child theme (Find it in Resources/js-child) for your customization so that you will not loose your changes when new version arrives.