WordPress.org

I don't really know CSS. Can I use this plugin?

You really do need to know CSS to get anywhere with this. Given the huge variety in styles provided by WordPress themes, it's impractical to attempt to predict what kinds of styles you might need.

The Custom styles I added aren't showing up in my blog -- why not?

Well, this is just a stab in the dark, but it's possible that the developer of your theme didn't use the WordPress function wp_head, which needs to be in your theme for this plugin to work.