You need to be using override.css as supplied in the override template. It's always loaded last. So if you use 1:1 same selectors as default template files it will override the styling. If you use weaker selectors then it won't. This is just standard CSS behavior. Your selector needs to be equal to or stronger than what you're trying to override. Example as follows.
Class:
.cb_template .my-class
Override Weak:
.my-class
In the above case the override will not replace existing styles of the current class. You will only be able to add new styles (e.g. a style not in the original class). Now if you use the following it'll work fine.
Override Same:
.cb_template .my-class
Override Strong:
.cb_template .parent-class .my-class
See the below on how to make an override template.
www.joomlapolis.com/forum/6-news-and-publicity/227459-cb-203-template-css-overrides
You can also use it as a starting point to make your own template. Each template file can basically be overridden individually (e.g. copy template.css, change it, and only that will override default).