Add a faithful Nord (dark) theme matching the official Nord palette

[jwendl]

Feature summary

Add a faithful Nord (dark) theme that matches the official Nord dark color scheme.

What problem are you trying to solve?

The theme picker already includes a theme called "Nord Midnight", but it doesn't actually match the official Nord color scheme — it's noticeably darker/bluer than real Nord and doesn't use the canonical Nord palette values. Users who love Nord (e.g. the popular VS Code Nord theme, which is the dark scheme) currently have no accurate option in the GitHub Copilot app, and there's no local theme file, config, or plugin hook to add one ourselves (themes are compiled into the app binary). So a built-in, true-to-palette Nord dark theme is the only path.

This request is specifically for the Nord dark scheme (the standard Nord theme with a Polar Night background) — not a light variant.

Reference: the official Nord theme and its exact palette are documented at https://www.nordtheme.com/.

Proposed solution

Ship a new built-in dark theme named "Nord" that uses the official Nord palette exactly as published on https://www.nordtheme.com/docs/colors-and-palettes. In the dark scheme, the Polar Night group provides the background/surfaces and the Snow Storm group provides the text. The palette is four groups of named colors:

Group	Token	Hex
Polar Night	nord0	#2E3440
Polar Night	nord1	#3B4252
Polar Night	nord2	#434C5E
Polar Night	nord3	#4C566A
Snow Storm	nord4	#D8DEE9
Snow Storm	nord5	#E5E9F0
Snow Storm	nord6	#ECEFF4
Frost	nord7	#8FBCBB
Frost	nord8	#88C0D0
Frost	nord9	#81A1C1
Frost	nord10	#5E81AC
Aurora	nord11 (red)	#BF616A
Aurora	nord12 (orange)	#D08770
Aurora	nord13 (yellow)	#EBCB8B
Aurora	nord14 (green)	#A3BE8C
Aurora	nord15 (purple)	#B48EAD

Suggested UI role mapping for the dark scheme (the typical convention used by Nord ports such as nord-visual-studio-code — Polar Night as background, Snow Storm as text):

Role	Nord token	Hex
App background	nord0	#2E3440
Elevated surface / sidebar	nord1	#3B4252
Selection background	nord2	#434C5E
Borders / muted UI	nord3	#4C566A
Secondary text	nord4	#D8DEE9
Primary text	nord6	#ECEFF4
Accent / focus / links	nord8	#88C0D0
Primary button	nord10	#5E81AC
Diff added / success	nord14	#A3BE8C
Diff removed / error	nord11	#BF616A
Modified / warning	nord13	#EBCB8B
Renamed / info	nord9	#81A1C1
Conflicted	nord12	#D08770

Example theme contents

For convenience, here's the same dark-scheme mapping expressed as CSS custom properties, in case it helps as a starting point for the theme definition:

/* Nord (dark) — official palette from https://www.nordtheme.com/ */
:root {
  /* Polar Night */
  --nord0:  #2E3440;
  --nord1:  #3B4252;
  --nord2:  #434C5E;
  --nord3:  #4C566A;
  /* Snow Storm */
  --nord4:  #D8DEE9;
  --nord5:  #E5E9F0;
  --nord6:  #ECEFF4;
  /* Frost */
  --nord7:  #8FBCBB;
  --nord8:  #88C0D0;
  --nord9:  #81A1C1;
  --nord10: #5E81AC;
  /* Aurora */
  --nord11: #BF616A;
  --nord12: #D08770;
  --nord13: #EBCB8B;
  --nord14: #A3BE8C;
  --nord15: #B48EAD;
}

/* Applied to UI roles — dark scheme: Polar Night background, Snow Storm text */
.theme-nord {
  --background-color:        var(--nord0);
  --surface-color:           var(--nord1);
  --selection-color:         var(--nord2);
  --border-color:            var(--nord3);

  --text-color:              var(--nord6);
  --text-secondary-color:    var(--nord4);

  --accent-color:            var(--nord8);
  --focus-color:             var(--nord8);
  --link-color:              var(--nord8);

  --button-background:       var(--nord10);
  --button-hover-background: #6a8fc0; /* nord10 lightened */
  --button-text-color:       var(--nord6);

  /* Status / diff colors */
  --color-new:        var(--nord14); /* added   */
  --color-deleted:    var(--nord11); /* removed */
  --color-modified:   var(--nord13); /* changed */
  --color-renamed:    var(--nord9);
  --color-conflicted: var(--nord12);
}

Workflow impact

Nord is one of the most widely used color schemes in developer tooling (official ports exist for VS Code, JetBrains IDEs, Vim, terminals, etc.), and the dark scheme is by far the most popular variant. A lot of developers keep a consistent Nord dark look across all their tools. An accurate Nord dark option lets those users match the GitHub Copilot app to the rest of their environment, reducing visual context-switching during long sessions. It also resolves the confusion that the existing "Nord Midnight" theme creates by implying Nord support that doesn't actually match the palette.

Additional context

• Official Nord theme website: https://www.nordtheme.com/
• Official palette reference: https://www.nordtheme.com/docs/colors-and-palettes
• Reference port for the dark-scheme role mapping: https://github.com/nordtheme/visual-studio-code
• Note: this is distinct from the existing "Nord Midnight" theme — the request is for a Nord dark theme that matches the official Nord palette values above.