Task tool get invisible second time

I am new to this, so I hope for some understanding if this is too trivial.
I have added BPMN IO to my web solution, and it works ok.
However, when clicking a task for the second (or more) time, the tool icons turn white and are not visible.
First time:

After that:

This seems to be an issue with your custom CSS. Can’t think of any other reason.

Thanks, I guess I will find the solution there… in my css’s