Linux puskom-ProLiant-DL385-Gen10 5.4.0-150-generic #167~18.04.1-Ubuntu SMP Wed May 24 00:51:42 UTC 2023 x86_64

/usr/share/themes/Ambiance/gtk-3.0/apps/

//usr/share/themes/Ambiance/gtk-3.0/apps/calendar.css

 /* Hide the white corners.  Calendar sets a background colour for the whole window. */
GcalWindow { background-color: transparent; }

/* Make the calendar focus ring appear around the selected day */
.calendar-view  {
    outline-style: dashed;
    outline-color: @selected_bg_color;
    outline-width: 2px;
    outline-radius: 8px;
    outline-offset: -4px;
}

/* Give pop-down lists the same selected bg colour as other menus. Fixes lp# 1558659 */
.calendar-list :hover {
    background-color: @selected_bg_color;
}