How do I change the background color of a Poster Subtitle?

Yes @krausjb, add this code to Design > Custom CSS. You'll need to change out the hex code for your color.

.sqs-block-image .design-layout-poster .image-subtitle p {
    background-color: #aa2727;
    box-shadow: 0.25em 0 0 #aa2727, -0.25em 0 0 #aa2727;
}

.sqs-block-image .image-block-outer-wrapper.image-block-v2 .image-subtitle p {
    display: inline;
    margin: 0;
    word-wrap: break-word;
    overflow-wrap: break-word;
}

awesome, thank you!