Introduction
There is a way to customize the checkbox using only css and whitout javascript.
Using the code
An Asp:Checkbox is composed of a label and an input type check (In html). So you need to use a plus (+) selector in css:
input[type=checkbox] { display:none; }
input[type=checkbox] + label:before {
font-family: FontAwesome;
display: inline-block;
}
input[type=checkbox] + label:before { content: "\f096"; }
input[type=checkbox]:checked + label:before { content: "\f046"; }
And that's it. You can use css classes just putting the class name first (something like .myCheck input[type=checkbox]...............). and then using it in CssClass property.
You must have this font to use it. You can use also image icons, just change content property, it can be whatever you want.