if you want to control the width of the container then just do this:
.mycustomclass { width: 200px; }
Again, if you master CSS selectors you will be able to figure this stuff out for yourself.
You can organize the CSS however you like in terms of which files you put things in, the only thing you have to keep in mind is that when they are combined they are combined in the sequence that the files are listed in style.config and this affects the specificity of a style rule. A style rule lower in the CSS will override a rule higher in the CSS.
You could create a new file mycustom.css and include it in the bottom of style.config if you want to.
Hope that helps,
Joe