gristlabs_grist-core/app/client/widgets/DateTimeEditor.css

.default_editor.celleditor_datetime {
  box-shadow: none;
  display: flex;
}

.celleditor_datetime_editor.celleditor_cursor_editor {
  flex: auto;
  min-width: 0;
  overflow: hidden;
  box-shadow: none;
  z-index: 9;
  outline: 1px solid var(--grist-color-cursor);
  position: relative;
}

.celleditor_datetime_editor:focus-within {
  box-shadow: 0 0 3px 2px var(--grist-color-cursor);
  z-index: 10;
  outline: none;
}

.celleditor_datetime_editor > .celleditor_text_editor {
  width: 100%;
}

.datepicker {
  color: var(--grist-theme-text, #333) !important;
  background-color: var(--grist-theme-menu-bg, #fff) !important;
  outline: none;
}

.datepicker-dropdown {
  box-shadow: 0 2px 20px 0 var(--grist-theme-menu-shadow, rgba(38, 38, 51, 0.6));
}

.datepicker .prev:hover,
.datepicker .next:hover,
.datepicker .datepicker-switch:hover,
.datepicker .day:hover,
.datepicker .month:hover,
.datepicker .year:hover,
.datepicker .decade:hover,
.datepicker .century:hover,
.datepicker th.today:hover,
.datepicker .focused
{
  background: var(--grist-theme-hover, #eee) !important;
}

.datepicker .active {
  color: var(--grist-theme-date-picker-selected-fg, #fff) !important;
  background-color: var(--grist-theme-date-picker-selected-bg, #286090) !important;
  border-color: var(--grist-theme-date-picker-selected-bg, #204d74) !important;
  text-shadow: none !important;
}

.datepicker .active:hover {
  background-color: var(--grist-theme-date-picker-selected-bg-hover, #204d74) !important;
  border-color: var(--grist-theme-date-picker-selected-bg-hover, #122b40) !important;
}

.datepicker .old,
.datepicker .new
{
  color: var(--grist-theme-text-light, #777) !important;
}

.datepicker .range-start,
.datepicker .range-end
{
  color: var(--grist-theme-text, #fff) !important;
  background-color: var(--grist-theme-date-picker-range-start-end-bg, #777) !important;
  border-color: var(--grist-theme-date-picker-range-start-end-bg, #555) !important;
  text-shadow: none !important;
}

.datepicker .range-start:hover,
.datepicker .range-end:hover
{
  background-color: var(--grist-theme-date-picker-range-start-end-bg-hover, #5e5e5e) !important;
  border-color: var(--grist-theme-date-picker-range-start-end-bg-hover, #373737) !important;
}

.datepicker .range
{
  color: var(--grist-theme-text, #000) !important;
  background-color: var(--grist-theme-date-picker-range-bg, #eee) !important;
  border-color: var(--grist-theme-date-picker-range-bg, #bbb) !important;
}

.datepicker .range:hover {
  background-color: var(--grist-theme-date-picker-range-bg-hover, #d5d5d5) !important;
  border-color: var(--grist-theme-date-picker-range-bg-hover, #9d9d9d) !important;
}

.datepicker td.today {
  color: var(--grist-theme-date-picker-today-fg, #000) !important;
  background-color: var(--grist-theme-date-picker-today-bg, #f7ca77) !important;
  border-color: var(--grist-theme-date-picker-today-bg, #f1a417) !important;
  text-shadow: none !important;
}
.datepicker td.today:hover {
  background-color: var(--grist-theme-date-picker-today-bg-hover, #f4b747) !important;
  border-color: var(--grist-theme-date-picker-today-bg-hover, #bf800c) !important;
}