[3.0] Theme split (wave 4, part 19) — point calendar.css at the design tokens

[albertlast]

Description

Part of the #7933 split, wave 4.
Calendar area.

index.css was converted when the design tokens landed. calendar.css was left behind and
still carried 33 hard-coded colours, so restyling the calendar means editing a
stylesheet rather than a variable, and dark.css and the colour variants have nothing to
reach for.

Seven of them already had a token elsewhere in the theme, so those just point at it:

was	now
#ddd (11 borders)	var(--window-border-color)
#f0f4f7 (2 table backgrounds)	var(--window-bg)

The rest get named tokens of their own, keeping the value they have now:

--calendar-th-bg:            #e7eaef;
--calendar-today-bg:         #fff;
--calendar-disabled-bg:      #eee;
--calendar-day-bg_hover:     rgba(97, 135, 166, 0.2);
--calendar-event-bg:         rgba(30, 245, 20, 0.1);
--calendar-event-bg_hover:   rgba(30, 245, 20, 0.2);
--calendar-holiday-bg:       rgba(23, 110, 245, 0.1);
--calendar-holiday-bg_hover: rgba(23, 110, 245, 0.2);
--calendar-birthday-bg:      rgba(102, 0, 255, 0.1);
--calendar-birthday-bg_hover: rgba(153, 51, 255, 0.2);
--calendar-link-color:       #999;
--calendar-link-color_hover: #555;
--calendar-title-link-color: #555;
--calendar-event-meta-color: #777;

Where this differs from the theme branch

It gives some of these different values — the header row follows the title bar instead of
being its own grey, today's cell takes the lightest primary tint, and holidays hang off
--primary-color-hue so a variant re-tints them. Those change how the calendar looks and
belong with that change, not with naming what is already there. The names match, so
re-pointing them later is a one-line edit each.

The event and birthday colours are the same colour in both, but written as hsla() there.
hsl(117, 92%, 52%) comes out as rgb(31, 245, 20), one off the rgb(30, 245, 20) in use,
so the literals stay for now.

Checked

The acceptance test for a token conversion is that nothing moves. Every rule in
calendar.css that sets a colour, a background or a border was applied to a probe element
and its computed values read, on both versions — 35 rules, 0 differences, including the
:hover and .calendar_today.events / .holidays / .birthdays rules that cannot be
sampled off the page directly. Computed styles on 18 elements of the month view are
identical too.

(The calendar needs #9405 to render at all on a stock install, so it was checked with that
merged in locally.)

calendar.rtl.css keeps its four white separator borders — there is no token that means
that, and the file is due to go when the calendar moves to logical properties.

Issues References (Fixes|Related|Closes)

Part of #7933

Co-Authored-By: live627 john@jbrock.us