The code I shared above broke recently. The class name changed, so the CSS selector no longer matched (I think it happened in #40001). This is the updated version I use on my forum now:
lowercase & replace spaces with hyphens
SCSS var name
.theme-card.=THEME_NAME= .theme-card-preview__image-wrapper svg {
display: none;
}
.theme-card.=THEME_NAME= .theme-card-preview__image-wrapper {
background-image: url($=UPLOAD_NAME=);
background-size: cover;
background-position: center;
}