It is relatively safe to change css as long as you keep track of what you are changing and knowing where to change it. If mantra has an option for custom css, your changes should be made there. If it doesn't, you should either use a custom css plugin or make a child theme and make your changes there.
An other thing which will be really useful is the built in Developer Tools in Chrome or Firebug. These tools will let you inspect your code and see change css on the fly.
To change the margins, you'll need to target your element either through its class or its id. If you provide a link to your site, I'll be able to tell you what they are. You can find that info by right clicking on the element in your page and choosing Inspect. Classes are targeted in CSS with a period in front of the class name (ex: .this-class) and id which are unique are targeted with a sharp symbol (ex: #this-unique-id).
In the end, if your content is being displayed in a div with id "content", your code would look like this
// You could also use the shorthand margin: 0 30px; to change top and bottom margin to 0 and left and right to 30px
And be placed in a custom css location.
I hope this helps you getting started with CSS.