Ok, no scss. Although it is against my better judgement, here is the quickfix:
Now your checkbox is clearly visible. Guess you will not be happy with the result though.
Thing is: you cannot style the checkbox itself from inside StudioPro, only its parent-div.
But without thinking about avoiding scss too much, give this a go:
.mx-checkbox {
input {
border: 1px solid black;
}
}
And watch how Mendix Studio Pro changes the border of your check box making it more visible;