Here’s a pared down chunk of the plugin’s CSS for the elements in the month/week grid views with the appropriate element selectors and just the background and color properties. I added comments to help identify what each selector is.
Go to Appearance > Customize > Additional CSS and paste in this block, then change the colors as needed. There are a number of tools online to help you identify RGB or HEX color values you can use in CSS, and you can also use HTML color names. You can find more about that here: https://rgb.to/html-color-names/1
/* All cells in the calendar grid */
.ics-calendar-month-grid th, .ics-calendar-month-grid td {
border: 1px solid rgb(187,187,187);
}
/* Day-of-week headers */
.ics-calendar-month-grid th {
background: rgb(171,171,171);
}
/* Days outside the currently selected month */
.ics-calendar-month-grid .off {
background: rgb(246,246,246);
color: rgb(153,153,153) !important;
}
/* Individual day cells */
.ics-calendar-month-grid .day {
background: rgb(226,226,226);
}
/* Day cells in the current month but in the past */
.ics-calendar-month-grid .past {
background: rgb(250,250,250);
}
/* Today's day cell */
.ics-calendar-month-grid .today .day {
background: rgb(102,102,102);
color: rgb(255,255,255);
}
/* Divider between individual items on the same day */
.ics-calendar-month-grid ul.events li {
border-bottom: 1px solid rgb(226,226,226);
}
/* Background for "all-day" events within a day cell */
.ics-calendar-month-grid ul.events li.all-day {
background: rgb(245,245,245);
}