Also, seems only IE10+ version is supported, wondering what prevents the library from working on IE9, since (I think) svg is supported on v9 as well.
The CSS is broken out into necessary structural bits (where everything is gray, there are no margins, &c.) and then into visual themes. If you aren't a designer, you can just use the stock theme. If you are a designer then you can start from the ground up to build the visual identity that you need.
At no point do you ever have to start throwing `!important` everywhere to override our styling decisions. We intend to release actual SASS/Compass mixins so that you can generate the structural CSS directly in your workflow instead of juggling multiple CSS files and their implicit relationships.
As for IE9, it's all flexbox issues. All of the charts are responsive, and Variance leans on flexbox CSS quite a bit (which IE9 does not support).