Wrong css background on extending page setting

I’m using OC 3.0.74 and when extending the Page CMS Settings popup according to this article:

I’m getting a weird grey background on the field that makes imposible to read the field content as the text is the same color as that background. Please see image:

As I was inspecting the css code, I saw it can be resolved setting:

.component-backend-inspector-panel .component-backend-inspector-controlhost .inspector-control.placeholder {
   background: transparent;

Thanks, @federico.schafer! This fix has been included in the next patch release.