Hi nikkoli,
For that, you will need to add some CSS to your Theme (for example) and for that, locate your theme under the Themes folder, under the Interface Tab and on the Style Sheet property add the following CSS:

[data-checkbox][disabled]:before, [data-checkbox][disabled]:checked:before {
border-color: black;
background-color: black;
}
[data-checkbox][disabled]:checked:after {
border-color: white;
color: white;
transform: none;
border-width: 0px 0px 2.5px 0px;
}
The value of your checkbox variable will have to be true in order to display the white dash.
You can change the 2.5px to more or less if you want the dash to be thicker or not.
Here's a glimpse on how it looks:

Hope it helps.
Regards,
João