How to Change Color Icon ?

It is not possible color these icons the easy way. The “heavy plus” looks great but can not be colored. But you can change it and then color it:

.simpletoc-collapsible::after {
    content: "✖";
    color: #e94c89;
} 

I aimed for a solution that looks good out of the box rather than to make is customizable. I will think about this in a future release. I am not happy with the implementation