the liste of all the drop-bos menus are black. Why. Can I modify the background

Based on your screenshot, I see that this is about form 60645160566355.

The color of your Drop Down menus is set inside the theme your form is using, and the theme sets background of the menu to black, while the text of the options is set to blue. Options are visible when your form is opened by regular users:

https://form.jotformeu.com/form/60645160566355

This is in Chrome:

And this is in Firefox:

The options are only not very readable when the form is opened inside the Form Builder for editing, because the Form Builder applies some of its own styles temporarily when you are editing the form. However, the options are perfectly visible by your end users - text is in bright color, while background is black.

If you need any further assistance, please let us know.