Location and mechanics of custom css

I want to make changes to the CSS for several EasyDiscuss modules and the main EasyDiscuss layout. However, there does not seem to be one central custom.css file for these types of changes, like there is with our theme from another company, which means I'm reduced to modifying each style.css for every module individually. That's a lot of work and will probably be overridden at the first update. I've read through many of the custom css posts, but most are bit over my skill level. I need a bit more step-by-step.

Here is any example. In the EasyDiscuss - Categories module, the category avatar displays with a border:

}

div.discuss-mod  img.avatar {

  border: 1px solid #ccc;

}

I would like the boarder to be 0px throughout all EasyDiscuss modules, not just categories. How do I accomplish that? FYI, I'm using the simplistic theme.