After pasting the default css of the theme and finding the "layout section" I tried to change the values for the margins.

You shouldn't copy and paste the entire original stylesheet unless you really know what you're doing and you select the appropriate option in the CSS Settings section. If you're not sure what to do, you probably want the "Add my CSS" option in the settings and then you'll want to *just* copy and paste the elements you want to change and not the entire stylesheet.

I can I remove the margins completely and make the website look decent without sidebar?

Here's some CSS to start with—it will hide the sidebar in the Quintus theme and expand the primary/content area to fit. The last part (#main) adjusts the margins, you can adjust those numbers to your liking.

#main .widget-area {
	display: none;
}

#primary,
#content {
	margin: 0;
}

#main {
	padding: 0 40px;
}

For the padding, if there are two numbers listed (like in the example above), the first number is the top and bottom padding and the second number is the left and right padding.

@msassone
The upgrade comes with Staff support. Please be patient while waiting to get help with your CSS editing.

Thank you very much