using knockout to set css class with an if condition

I want to set a bootstrap css class to a span with if condition (up to the binded value). I have isApproved field in a list, I want to see the field with 'label-success' class when its active and 'label-important' class when its inactive

I added this line, but all the time it's taking the first class

data-bind="
    text: isApproved,
    css: isApproved = 'true' ? 'label label-important' : 'label label-important'"

Is it possible in the html or I should add a computed field on my VM?