How to… change the colour of the Roll-over on simple post box (with featured image)

Hello Gianluca !
To change color on the post box when you hover, you can add the following code to the header code (Dashboard > Appearance > Customize > Custom Code)

 <style>
.block .view-pic .mask, .block .view-product .mask {
    background-color: #...;
}
.block .view-pic:hover .mask, .block .view-product:hover .mask, .block .view-video:hover .mask {
    opacity: 0.50;
}
.block .view-video:hover .mask {
    background-color: #...;
}
</style>

– After change color for posts box, you can change the content color for posts box, please add the following code to the Header Code:

 <style>
.block .mask-content .block-title {
     color: #...;
}
.block .mask-content .block-meta {
   color: #...;
}
.block span.line {
    background:  #....;
}
</style>

Hope this helps !