Aion Theme

Aion

A dark theme with a measured contrast floor. Familiar syntax, distinct chrome.

Syntax follows the One Dark Pro role map, so muscle memory survives. Identity comes from the base, the gold chrome and the structure.

Two things make Aion different

The editor is the darkest surface. The sidebar, the activity bar and the status bar sit one step above it, and the panel sits between the two. Your code is the deepest thing on the screen. Of the four themes compared below, two put the sidebar under the editor and two use one colour for both.

The contrast floor is a gate, not a claim. Every colour is defined in OKLCH and checked against the surface it sits on. npm run verify exits non-zero on a colour below 4.5:1, and on a syntax colour that falls below it under a current line, a selection, a word highlight or a diff fill. CI runs that gate on every branch, so no release can carry a colour under the floor. Those decorated states are the part no comparison below covers.

Measured across the same eight syntax roles, against each theme's own editor background, on a plain editor line:

Theme	Lowest ratio	Below 4.5:1	Source
Aion	6.95	none	—
One Dark Pro	3.73	comment, variable	One Dark Pro @ `54c3280`
Ayu Mirage	3.42	comment	Ayu Mirage @ `444ef92`
Nord	2.43	comment, number	Nord @ `8ead098`
Catppuccin Mocha	5.81	none	Catppuccin Mocha @ `befc9e6`

This compares eight syntax roles on one background. It is not a measure of a theme's accessibility or of its usability, and two of the four clear the floor on this test. The values were read from each definition on 2026-09-05, at the revision named. A theme changes: One Dark Pro's comment and Catppuccin's comment both moved between this table and the one before it, which is why the revision is recorded.

The palette

	Hex		Hex
editor	`#11151c`	coral, variables	`#ed807e`
panel, terminal	`#14181f`	copper, numbers	`#ea955f`
sidebar, status bar	`#171b22`	gold, types	`#e4c058`
widget, menu	`#1e222a`	green, strings	`#7bce88`
primary text	`#e2e8f3`	teal, operators	`#59cdc8`
secondary text	`#a7acb7`	blue, functions	`#7db2f7`
comment	`#98a0af`	violet, keywords	`#bd96e9`

Gold is the signature: the cursor, the focus ring, the search hit and the active tab marker. Violet is a syntax hue. It reaches five interface keys and no others: the two bracket-cycle steps, the keyword symbol icon that mirrors the syntax colour, charts.purple where an extension asks for the hue by name, and ANSI slot 13.

What is inside

Every interface key in the table below. VS Code still falls back to its own default for any key a theme does not set.
TextMate rules and semantic tokens, kept in step with each other.
Language overrides for Markdown, JSON, YAML, HTML, CSS and JSX/TSX.
Bracket pairs that nest gold → teal → violet.
A terminal whose bright eight are byte-identical to the syntax accents.
No italics. Not on comments, not on keywords.

Matching terminal

The same sixteen ANSI slots ship for Windows Terminal, as a fragment extension that needs no settings edit. See the repository.

What the contrast floor covers

Every syntax colour clears 4.5:1 on the editor, the peek editor, a hover or suggest widget, and under a current-line highlight, a selection, a word highlight, or a stack of the two the renderer can draw together. It clears it again with a diff line wash, or a line wash and a word wash, painted on top of a plain line, a current line or a selection. A find match keeps the colour under it: VS Code applies each of its two foreground override keys to the other one's decoration, so the fills carry every syntax colour on their own.

What the gate checks	Count
Rows measured	611
Below their floor	0
Exempt rows, all documented	5
Reading states per syntax colour	35
Lowest ratio in a reading state	4.50:1
Interface keys the theme sets	622
TextMate rules	64
Semantic tokens	32

The set is exact rather than approximate. §3.1 of DESIGN.md names every state and the foreground that reads worst on it. A state outside that table is outside the claim: the current line and a selection are not stacked, because VS Code draws the current-line background only while every selection is empty, and a diff wash is measured on a plain line, a current line and a selection rather than on every decoration in the editor.

Three kinds of exemption are named rather than hidden:

The inactive line number sits at 4.25:1. It is decorative.
Hairline and divider borders sit below 3:1. They separate regions that already read as separate; the edges of controls do not use them.
ANSI slot 0 reads 1.31:1 as a foreground. SGR 30 does select it, so that is a real limitation and not a claim that no application uses it. No dark scheme can fix it and keep slot 0 usable as a background, which is what SGR 40 and reverse video need. What slot 0 does guarantee is the other direction: ANSI white reads 5.98:1 on it and bright white 11.07:1.

Colour vision

Success and error separate by 0.06 in OKLCH lightness as well as by hue, and so do the two diff gutter strips. A test asserts that gap on the pair that can hold it: the strips are opaque, always drawn and never covered. The line washes cannot hold it and do not claim to — both are lighter than the editor and about 0.02 apart, because a green wash dark enough to open a 0.06 gap under a red one is invisible.

That is a palette invariant, not a claim of deuteranopia usability: it has not been checked with a simulator or with users. Diff lines and problem markers carry a gutter glyph and an icon as well as a colour, so the meaning does not rest on hue alone.

Licence

MIT.

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