There’s a few ways actually, directly, or adding a new style, or doing inline styling. The styling for this is found in the style.css file around line 1526 and looks like this:
.thumbnail {
display: block;
padding: 0;
line-height: 20px;
background-color: #EFEFEF;
}
If using a child theme or a CSS editing plugin, you can simply add a new override style like this:
.thumbnail {background-color: #ffffff;}
Or you can add an inline style directly into the content code of your editor when you make these boxed images like this:
The benefit of the last one is that you can have different colours for each box, or different pages and posts when used.