Well spotted! I think I just concentrated on ensuring that the blue background worked!
Try:
.planner {
th {
background: lightgrey !important;
}
tr:nth-child(odd) {
background: lightblue;
font-weight: bold;
}
}
There's a bit more information about using custom css in Joplin here.
At this time custom css is limited to the desktop clients. I do not know if it would even be possible to implement on the mobile clients (I am not a dev just another user).