[3.0] Theme split (wave 5, part 2) — give the default theme a dark mode

[albertlast]

Description

The colour mode plumbing has been in 3.0 since Alpha. Theme.php resolves the mode, Themes.template.php and ThemeOptions.php offer it to the admin and the member, index.template.php writes it into data-mode, and dark.css has sat in the theme as a placeholder waiting to be filled.

None of it has ever run. Turning the flag on is not enough, because the feature is broken in four places rather than one. Each of these was found by trying to use it, not by reading it.

Theme::loadMode() had no callers	It is the method that resolves the mode and loads dark.css. Nothing anywhere invoked it, so data-mode was never emitted and the stylesheet never requested. Now called beside loadVariant(), its sibling, which the same line of the template reads.
system lost the thing that makes it system	That mode works by loading dark.css with a (prefers-color-scheme: dark) media attribute. But attributes are only printed for files served on their own, loadCSSFile() defaults to minimizing, and a file folded into the bundle loses them — so the browser was never asked and system would have meant always dark. That mode now opts out of the bundle. light and dark carry no attribute and still share one cached file.
Saving the mode to a profile was fatal	Db::insert() wants a list of rows and was handed a single flat one. Outside backward-compatibility mode that is a critical error, so the first time a logged-in member picked a mode the page died with Invalid data structure sent to the database.
dark.css was empty	Filled in — see below.

The stylesheet

Written against release-3.0's own tokens rather than ported. The snapshot on #7933 could not be used: of its 197 overrides only 84 name a token that exists here, while 213 colour-bearing tokens here have no value in it. Taken as-is it would have produced a dark mode with most of the forum still showing its light values.

It overrides 243 tokens and restates no rules. Everything is written against twelve values declared at the top of the file — five surfaces, two hairline weights, three text weights and two link colours — so the way to review this is to agree those twelve and then check that each component reached for the right one.

The primary and secondary ramps are deliberately left untouched. They carry the theme's identity and a variant re-tints the forum by moving --primary-color-hue; leaving them alone means dark mode moves with it. Where a token pointed at the dark end of a ramp because it was drawn on a light surface, the override points it at the light end instead, rather than redefining what the ramp means.

Verification

Run against a real forum, not read.

• Light mode is unchanged. Computed color, background-color, background-image, all four border colours, box-shadow, outline-color and text-shadow for every element on twelve pages, with the file present and absent: 4254 elements, 0 differences.
• All three modes emit correctly, including the media attribute on system and on system alone.
• Eighteen pages swept in both modes for text landing on a background too close to it — board index, topic, posting form, profile, profile theme options, stats, help, search, calendar, memberlist, personal messages, moderation centre, admin centre, its permission grid and a group's permission form, the theme list, maintenance and the error log. Dark mode has 19 elements below the AA ratio against light mode's 136.
• No errors logged during the sweep.

Known gaps, both outside the theme

The 19 are two causes, neither of them a token:

• The bundled coloris colour picker (coloris.min.css) is third-party and carries no tokens at all, so its Hex/RGB/HSL labels stay light-on-light. Same class of thing as .sceditor-container — not ours to name.
• The Administrator group's online_color is #FF0000, set by the installer. That is data, not styling, and it is marginal in light mode too.

Depends on #9496 (wave 5, part 1), which gives the last hard-coded colours in index.css a token. Without it #wrapper keeps a white background, the .titlebg headings stay #555 on a dark bar and the three sheen gradients stay pale. This branch is stacked on it, so the diff here is dark.css, Theme.php and one flag.

loadVariant() carries the identical Db::insert() defect three lines below the one fixed here. It is left alone deliberately: this branch neither enables nor exercises variants, and shipping a fix I cannot verify is worse than recording a known one.

Part 2 of wave 5 of the #7933 split.

Issues References (Fixes|Related|Closes)

Related #7933