pop-up (defaut storyline) background

Hello everybody,

I'm working on a module where I don't like pop-up from storyle defaut appear.

I have a quiz on which learner has to type their answer (in some questions text and other numbers). I change my customized submit button to normal state when learner types any key, in order tavoid the invalid answer message appears. However, if learner erases their answer s/he can click on submit button and then that defaut invalid message appears. the problem is that in that case I would like to have a background with some opacity behind that pop-up, I haven't found a way to do so. Has someone ever tried this ? do you know if it is possible?

thanks in advance