After playing around with the CSS for a while, I figured out what to change.
/* Dashboard Tiles */
.dashboard {
background-color: #1b1e2c !important;
}
/* Dashboard Tile Background (match the main background color) */
.tile {
background-color: #24283b !important;
}
/* "stopgap" (horizontal space in between arrangeable tiles) */
.stopgap {
background-color: #141620 !important;
}
Mind you, I'm pointing to a locally hosted css file that I customized from the Dracula theme. Hardly know CSS, so don't know if this is "proper" but it works for me.