Hi,
You can override the css in a custom stylesheet in your campaign or template. Click in the drag & drop editor in the right panel on the cog wheel. There you will find a button to add a custom stylesheet.
Be advised to add "!important" to all your css to make sure your css does not get overridden by the default css.
In one of the next releases Acyba will add a settings panel where you easily can add your default css.