How can I customize the style of a checkbox which is in indeterminate state?

So I'm trying to customize my checkbox so that it will look like this in indeterminate state. The checked and unchecked work fine, but I can't make the indeterminate one show up properly. Any help would be greatly appreciated!
I have tried tag.class:indeterminate {} and input[type=checkbox]:indeterminate {} in the style sheet but it doesn't seem to work.

My code and CSS sample: Link