Add a Stevenson green and gold theme

[ShankarK2009]

Adds a Theme picker to Settings → Appearance, next to the existing color scheme and time format controls. Stevenson is the default; Classic restores the stock iOS palette the app shipped with.

Modelled on how stevenson.space handles the same two colors: a green hero band, and an accent that changes with the appearance.

The accent swaps rather than being picked

Contrast rules out any single accent:

pair	ratio	verdict
#1B5E20 on white	7.9:1	AAA
#1B5E20 on a dark background	1.6:1	unusable
#B38825 on white	3.3:1	fails AA for text
#C9992B on a dark background	6.5:1	AAA

Green reads on white and dies on black; gold does the reverse. So the app accents in green in light mode and gold in dark mode, and neither appearance is asked to carry a color that cannot hold text there.

The hero band

The day header and the countdown now sit in a field of school green, curved off where the day's cards begin — extended to the weekend, break and summer screens too, so a non-school day gets a real header instead of an icon floating on grey.

Dark mode keeps a green band rather than a neutral one: deep enough to read as a dark surface (1.5:1 against black) while never letting the screen go grey.

The countdown gets a disc punched out of the band, matching the system appearance so .primary/.secondary stay correct on it. The ring is pulled well inside the disc — the arc is school green, and school green on the green band is the same color twice over. The track takes whichever school color the arc isn't wearing, so the ring reads as green and gold in both appearances.

The light gold

Brand gold clears only 2.4:1 against green, which rules it out on the band entirely. Lightening it to #E8C560 reaches 4.7:1 — that one value is what lets gold appear on green at all, and it carries the status-screen glyphs and the header badges.

Semantic colors stay semantic

Late Arrival purple, Early Dismissal red, passing-period orange, the override badge and destructive red are information, not branding, and are untouched in both themes. Where the band would swallow one, the fix reinforces its shape rather than repainting it:

• An Early Dismissal capsule clears only 2.2:1 against green, so it keeps its red fill and gains a white border.
• The passing-period banner keeps its orange on an opaque card, since a 12% wash would vanish into the green.

Both resolve to the old rendering under Classic.

Notes for review

Two judgment calls worth a look:

1. Stevenson is the default, including for existing installs — a config blob written before themes existed decodes as Stevenson rather than pinning upgraders to Classic. This is the Stevenson app, so the school identity seemed like the right default, but it's a one-line change if you'd rather ship Classic as the default.
2. The ID and Lunch tabs now follow the theme. Their accents were previously hardcoded green regardless of any preference, which meant two tabs were already half-themed. Routing them through the theme makes the picker actually mean something — the side effect is that those tabs go system blue under Classic.

The ID card deliberately keeps its own crest-sampled palette. It emulates printed stock, doesn't follow dark mode, and its lighter print gold carries ink at 7.0:1 versus the brand gold's 5.6:1 — moving it onto the brand values would have cost real legibility. The two greens are close enough to read as the same school green.

Verification

• swift test on both packages: 185 + 103 tests pass, including a new assertion that a pre-theme config blob decodes as Stevenson and that the preference round-trips.
• Checked in the simulator in light and dark, both themes, across mid-class, passing period, weekend, and Early Dismissal / Finals states. Classic renders identically to main in each.