ui: phosphor theme (80s CRT green)

Adds a centralized color theme with two presets and a default-on
phosphor-green palette. Errors stay red and warnings stay yellow
across every preset — the semantic urgency channel doesn't get
traded for aesthetics.

## New module

`src/ui/theme.rs` — `Theme` struct with semantic roles (agent, user,
system, tool, perm, result, error, warn, accent, dim, header,
divider, banner_*) and two presets:

- `phosphor` (default) — green accents on black for the assistant
  chat, dim green for results/tool output, bright green for headers/
  banner. Permission prompts use yellow so they stand loud.
- `plain` — the pre-theme white assistant / cyan accent look. Set
  `"theme": "plain"` in config.json if green-on-black doesn't suit
  your terminal background.

The active theme is initialized once at boot from `cfg.theme` and
read everywhere via helper fns (`theme::agent()`, `theme::error()`,
…) so future palette swaps don't ripple across call sites.

## Welcome banner

`render_session` now opens with a 4-line CRT-style box-drawn banner:

  ╔══════════════════════════════════════════╗
  ║ ░ DIRGE ░ PHOSPHOR ░ v1.0.0              ║
  ║ provider: openai · model: gpt-4          ║
  ╚══════════════════════════════════════════╝

Truncates gracefully on narrow terminals (clamps inner width to
`terminal_width - 4`).

## Call-site migration

The 213 references to `C_AGENT` / `C_ERROR` / `C_TOOL` / `C_PERM` /
`C_RESULT` constants in `src/ui/mod.rs` + `src/ui/slash.rs` are now
`c_agent()` / etc. fn calls that delegate to the theme module. Spelling
stayed similar so the diff is mechanical search-replace plus tiny
shims at the top of each file.

`renderer.rs` panel + status + input-prompt color literals (`Color::Cyan`,
`Color::DarkGrey`, `Color::DarkYellow`) replaced with theme accessors
(`accent()`, `dim()`, `warn()`). Picker (file picker + list picker)
got the same treatment.

`events.rs::render_session` now uses themed colors for user `>` /
assistant `<` / system `#` prefixes (`theme::user/agent/system`).
The leading `░` glyph on AGENTS.md/compaction status lines gives the
banner aesthetic a little continuity into the chat scrollback.

## Config

New `theme: Option<String>` field on `Config`. Documented in CONFIG.md
+ a short note in README.md. CLI flag intentionally omitted — palette
is a long-lived per-user preference, not a per-invocation choice.

## Tests

3 new in `ui::theme::tests`:
- `presets_are_distinct`
- `error_and_warn_stay_loud` (locks in the red/yellow contract for
  every shipping preset)
- `init_with_unknown_name_falls_back`

Totals:
- [x] `cargo test --features plugin` -> 599 pass, 0 fail (was 596 + 3).
- [x] `cargo build --features plugin` -> 0 warnings.
- [x] `cargo build` -> 0 warnings.
- [x] `./target/debug/dirge --version` works.

Author: Yogthos

CONFIG.md

@@ -81,6 +81,7 @@ Accepted top-level keys:
 | `show_edit_diff`          | boolean | Show colorized diff output for `edit` tool results (`-` red, `+` green, `@@` cyan). Default: `true`.                                                                        |
 | `tool_result_max_chars`   | integer | Maximum characters to show before truncating tool output with `[N more chars]`. Default: `500`.                                                                              |
 | `default_prompt`          | string  | Prompt name to activate on startup. Default: `code`.                                                                                                                        |
+| `theme`                   | string  | UI color theme. `phosphor` (default — 80s CRT green-on-black) or `plain` (pre-theme white/cyan). Unknown values fall back to `phosphor` with a warning.                       |
 | `mcp_servers`             | object  | MCP server map when compiled with the `mcp` feature. When omitted, defaults to a single Exa Web Search server; see below.                                                   |
 | `acp_servers`             | object  | ACP server config map when compiled with the `acp` feature. See the ACP section below.                                                                                       |
 | `acp_host`                | string  | TCP bind host for ACP server mode (equivalent to `--acp-host`).                                                                                                              |

README.md

@@ -301,6 +301,16 @@ Session allowlists persist approvals for the session. Doom-loop detection trigge
 
 See [CONFIG.md](CONFIG.md) for config file location, accepted keys, provider aliases, permission rules, and MCP server configuration.
 
+### UI theme
+
+dirge ships with an 80s-CRT phosphor green palette by default. To opt out, set `"theme": "plain"` in `config.json` for the pre-theme white/cyan look:
+
+```json
+{ "theme": "plain" }
+```
+
+Errors stay red and warnings stay yellow under every theme — those colors are part of the load-bearing semantic contract.
+
 ## Supported providers
 
 - OpenRouter (default)

src/config/mod.rs

@@ -104,6 +104,10 @@ pub struct Config {
     pub show_edit_diff: Option<bool>,
     pub tool_result_max_chars: Option<usize>,
     pub default_prompt: Option<String>,
+    /// UI color theme. Known values: `phosphor` (default, 80s CRT
+    /// green) and `plain` (the pre-theme white/cyan look). Unknown
+    /// values fall back to `phosphor` with a warning.
+    pub theme: Option<String>,
     pub tools: Option<ToolsConfig>,
     #[cfg(feature = "lsp")]
     pub lsp: Option<LspConfig>,

src/main.rs

@@ -194,6 +194,10 @@ async fn main() -> anyhow::Result<()> {
 
     let cli = cli::Cli::parse();
     let cfg = config::load();
+    // Initialize the global UI theme before any rendering happens. The
+    // theme is global state; setting it once at boot keeps every
+    // render site from having to thread it explicitly.
+    ui::theme::init(cfg.theme.as_deref().unwrap_or("phosphor"));
     let mut context = context::load(cli.resolve_no_context_files(&cfg));
 
     let default_prompt = cfg.default_prompt.as_deref().unwrap_or("code");

src/ui/events.rs

@@ -8,6 +8,7 @@ use crate::context::ContextFiles;
 use crate::session::{MessageRole, Session};
 use crate::ui::markdown;
 use crate::ui::renderer::Renderer;
+use crate::ui::theme;
 
 pub fn format_time(rfc3339: &str) -> CompactString {
     let dt = chrono::DateTime::parse_from_rfc3339(rfc3339).ok();
@@ -40,38 +41,31 @@ pub fn render_session(
     } else {
         cli.resolve_model(cfg)
     };
-    let welcome = format!(
-        "dirge {}  {}  {}",
-        provider,
-        model,
-        env!("CARGO_PKG_VERSION")
-    );
-    renderer.write_line(&welcome, Color::Cyan)?;
-    renderer.write_line("", Color::White)?;
+    render_banner(renderer, &provider, &model)?;
     if context.agents.is_some() {
-        renderer.write_line("loaded AGENTS.md", Color::DarkGrey)?;
-        renderer.write_line("", Color::White)?;
+        renderer.write_line("░ loaded AGENTS.md", theme::dim())?;
+        renderer.write_line("", Color::Reset)?;
     }
     if !session.compactions.is_empty() {
         renderer.write_line(
             &format!(
-                "compacted {} times (saved ~{} tokens)",
+                "░ compacted {} times (saved ~{} tokens)",
                 session.compactions.len(),
                 session
                     .compactions
                     .last()
                     .map(|c| c.token_savings)
                     .unwrap_or(0),
             ),
-            Color::DarkGrey,
+            theme::dim(),
         )?;
-        renderer.write_line("", Color::White)?;
+        renderer.write_line("", Color::Reset)?;
     }
     for msg in &session.messages {
-        let (prefix, _c) = match msg.role {
-            MessageRole::User => (">", Color::Green),
-            MessageRole::Assistant => ("<", Color::White),
-            MessageRole::System => ("#", Color::DarkGrey),
+        let (prefix, line_color) = match msg.role {
+            MessageRole::User => (">", theme::user()),
+            MessageRole::Assistant => ("<", theme::agent()),
+            MessageRole::System => ("#", theme::system()),
         };
         if msg.role == MessageRole::Assistant {
             let max_width = renderer.line_width();
@@ -84,14 +78,63 @@ pub fn render_session(
             }
         } else {
             for line in msg.content.lines() {
-                renderer.write_line(&format!("{} {}", prefix, line), _c)?;
+                renderer.write_line(&format!("{} {}", prefix, line), line_color)?;
             }
         }
-        renderer.write_line("", Color::White)?;
+        renderer.write_line("", Color::Reset)?;
     }
     Ok(())
 }
 
+/// 80s-CRT welcome banner. Four lines: top border, wordmark + theme +
+/// version, provider/model summary, bottom border. Width clamps to the
+/// terminal so narrow windows don't see overrunning box-drawing chars.
+fn render_banner(renderer: &mut Renderer, provider: &str, model: &str) -> anyhow::Result<()> {
+    let label = theme::current().label;
+    let version = env!("CARGO_PKG_VERSION");
+    let title = format!("░ DIRGE ░ {} ░ v{}", label, version);
+    let subtitle = format!("provider: {} · model: {}", provider, model);
+    // Inner width = max content length + 2 padding on each side.
+    // Clamp to terminal width − 4 (margin) so we don't push the banner
+    // past the visible region.
+    let term_w = renderer.line_width().max(20);
+    let max_inner = term_w.saturating_sub(4);
+    let inner = title
+        .chars()
+        .count()
+        .max(subtitle.chars().count())
+        .min(max_inner);
+    let inner_width = inner + 2; // single-space padding each side
+
+    let border_top = format!("╔{}╗", "═".repeat(inner_width));
+    let border_bot = format!("╚{}╝", "═".repeat(inner_width));
+    let title_line = format!("║ {:width$} ║", truncate(&title, inner), width = inner);
+    let sub_line = format!("║ {:width$} ║", truncate(&subtitle, inner), width = inner);
+
+    renderer.write_line(&border_top, theme::banner_secondary())?;
+    renderer.write_line(&title_line, theme::banner_primary())?;
+    renderer.write_line(&sub_line, theme::banner_secondary())?;
+    renderer.write_line(&border_bot, theme::banner_secondary())?;
+    renderer.write_line("", Color::Reset)?;
+    Ok(())
+}
+
+/// Truncate a string to `max` *characters* (not bytes), adding an
+/// ellipsis when shortened. Used by the banner so wordmarks stay
+/// inside the box drawing.
+fn truncate(s: &str, max: usize) -> String {
+    let count = s.chars().count();
+    if count <= max {
+        s.to_string()
+    } else if max <= 1 {
+        s.chars().take(max).collect()
+    } else {
+        let mut out: String = s.chars().take(max - 1).collect();
+        out.push('…');
+        out
+    }
+}
+
 pub fn sanitize_output(text: &str) -> CompactString {
     let mut result = String::with_capacity(text.len());
     let mut chars = text.chars();