How to improve comments when using Acquia Marina theme
The only options for changing the styling of the comments in Acquia Marina are to customize the theme or to install a third-party commenting system such as Disqus http://drupal.org/project/disqus

If you decide to customize the styling of Acquia Marina, here are a few tips and resources:

Documentation on customizing a Fusion-based theme. Be sure to read the section on "Using local.css" http://fusiondrupalthemes.com/support/documentation/customizing-your-theme

If you are not very familiar with CSS, a good, general tutorial on what CSS is and how to use it - http://www.w3schools.com/css/default.asp

The Firebug plugin for Firefox is very helpful for troubleshooting CSS and discovering what CSS rules need to be written to make changes to your theme's styling. Here is a tutorial on using that plugin: http://www.kristarella.com/2009/02/how-to-use-firebug-for-css/