colorize the background of days by status of s calendar entry

[fugazzy]

Hello,
I would like to colorize the background of the days where a specified calendar entry is active.
E.g. where the day is a holiday day.

[FamousWolf]

You can already do that with card-mod:

card_mod:
  style: |
    .day:has(.event[data-entity="calendar.personal"]) {
      background-color: #ff0000;
    }

Where calendar.personal is the calendar entity. Any day with an event from that calendar will get a (in this case) red background.