When I open it with an editor it looks like this, there is no code.
dasilvaamandio: You click on Appearance > Editor > styles.css (or whatever css file is provided with your theme – there is often more than one css file with a theme)
As it warns you in your posted image any changes you make there will be overwritten with future updates so you should make a Child Theme for your own css…
They moved the CSS code to another file. All they need to do is change index.php, say add a <link> line.
Anyway, as Duncan says, if you put rules in their style.css, they will gone during the next theme update. Create a child theme first.
Also, use the Developers console / Firebug to find out where styles are defined. Just right-click any part of the website and select "Inspect element" from the context menu.
You should use the Child Themes feature. Follow the instructions Duncan linked to. Overriding CSS is pretty easy: use the same selector, and whatever style declarations you want. There are ways to undo styles as well, using the values like "default" or "inherit" for properties.
View the page source, then connect via SSH or FTP, to the server, download the CSS, or edit in place (make a backup before) then edit and save.. Reload the webpage.
Mono develop and unity script editor or any script editor like mono dev.