Re-styling a checkbox in bootstrap 3

I'm having a real difficult time trying to style a checkbox in bootstrap, I currently have the default checkbox, and I need to style it to look like this I'm aware this picture it's round but the designer made a mistake, so instead of being round it still needs to be square

I have looked at the following and also tried what is suggested.

Twitter Bootstrap radio/checkbox

I did on the other hand find a website which has a similar to style to what i'm trying to achieve which is on here located on the left hand side where you do the filtering

Example of the checkbox

I tried using firebug to get/check out the CSS but I was unable to obtain the CSS.