Style applied for a checkbox inside a generated unbreakable html

I searched for my question hours on the internet but I couldn't find any example that would work (or at least without change the html code).

Basically I just want to replace the typical common checkbox with a new stylish blue one, like this:

What I have is a bunch of old php functions (yea) that basically generates the input elements, in this case the checkbox. Cannot add here all the code, and this is not my purpose, to change that behaviour. I just want to have at least something similar with this.

The html generated that I can see in inspects is:

So what I said and what can we see from the 2nd picture is 'whaaat'? The only css that was applied for the checkbox was:

 input[type="checkbox"] {
   margin-right: 12px;
 }

Now, I have no idea what should I do in order to transform the basic checkbox in the desired one, in respect with all of the generated html code. If someone can point me in a good direction, but please, without telling me to modify the html code because it's a killing option, would be very appreciated.

Thanks