[3.0] Theme split (wave 4, part 27) — point the quick buttons at design tokens

[albertlast]

Description

Part of the #7933 split, following #9467 and #9468.

The quick button strip and the drop down that hangs off it were the next group of hard-coded values in index.css. .button itself has read --button-* since wave 2; these are the variants that sit on top of it — the strip's own colour, the four corner radii that make the buttons read as one bar, the moderation checkbox focus ring, and the post options drop down.

24 tokens in three groups, placed after /** Buttons **/, so this does not collide with #9467, #9468, #9443 or #9448. Every token holds the value the rule has today, so nothing renders differently.

The drop down sets a border and then overrides its left and top edges, so those keep their own tokens and all three stay shorthands.

One naming call worth flagging. The hover colour is --quickbutton-color_hover even though .button:hover is listed in the same selector:

.button:hover, .button:focus,
.quickbuttons > li:hover > a, .quickbuttons > li > a:focus {
	color: var(--quickbutton-color_hover);
	...
}
.button:hover, .button:focus {
	color: var(--button-color_hover);
}

The rule immediately below puts --button-color_hover back for .button, so #222 only ever reaches a quick button. Naming it after the button would be wrong about what it does.

Verification

0 differences, by both methods:

• Raw source: index.css fetched over HTTP, rule bodies parsed out of the text and applied to a probe in cascade order — including :first-child, :last-child, :only-child, :hover and :focus, and the .button override chain above. 11 chains x 39 longhands = 429 values, 0 differences.
• Real elements, in iframes at 1280px, on the three pages that actually draw these: ?action=recent, ?action=pm and the reply form's previous-posts list. 30 elements, 1200 values, 0 differences.

Rule text is taken from the file rather than from CSSRule.style.cssText, which cannot round-trip a border shorthand holding a var() that is then overridden on one edge — see #9468 for what that looks like when it goes wrong.

Noted while finding elements to measure, not addressed here: a topic display draws no .quickbuttons at all, logged in as an admin — they appear on the recent-posts list, the PM index and the reply form, but not under a post on ?topic=.

Issues References (Fixes|Related|Closes)

Related: #7933, #9467, #9468

[albertlast]

Correction to the last paragraph of the description: the claim that a topic display draws no .quickbuttons is wrong. I measured that on ?topic=1.0, which is a fatal on release-3.0 — Object of type SMF\TimeInterval (inheriting DateInterval) has not been correctly initialized by calling parent::__construct(), which is #9405 and still unmerged. There were no quick buttons because there was no post; the error page has none of the topic markup at all.

The verification in this PR is unaffected: the raw-source cascade chains were captured on ?board=1.0 and the 30 real elements in iframes on ?action=recent, ?action=pm and the reply form, all of which render.