System theme responsiveness

[mdrxy]

Privileged issue

• I am a Deep Agents maintainer.

Issue Content

e.g. set a default light/dark mode theme, responsive to system theme

alternatively, assign a theme depending on the terminal used/some env var

Upstream:

• Automatic choice of dark/light theme based on terminal's colours Textualize/textual#2419
• Support the terminal color palette update notifications protocol (terminal dark/light mode detection) Textualize/textual#6151

Area (Required)

• deepagents (SDK)
• code
• Other / not sure / general

[mdrxy]

Relevant for VS Code integrated terminal detection:

VS Code sets TERM_PROGRAM=vscode (and TERM_PROGRAM_VERSION) in its integrated terminal. This could be used as one of the env var signals for per-terminal theme assignment — e.g., auto-selecting a theme that complements VS Code's active color scheme.

Other terminals set similar variables (TERM_PROGRAM=Apple_Terminal, iTerm.app, WezTerm, tmux, etc.), so a mapping of TERM_PROGRAM → default theme could cover many common cases alongside the Textual dark/light mode detection from the upstream issues.

[mdrxy]

https://x.com/kmdrfx/status/2036235877844734171

[mdrxy]

Open to external contributions

[yuvrxj-afk]

Interested in picking this up.

The TERM_PROGRAM mapping fits cleanly into _load_theme_preference as a [ui.terminal_themes] table between the env var and the saved theme. No upstream dependency, ships any time.

For the system dark/light side: PR #6151 adds a TerminalColorTheme message and an App.terminal_light_dark_mode attribute to Textual itself, so on our side it's a small subscription once that lands and we bump Textual. The PR's been waiting on review since November '25 though — if you'd rather not block on that, we could DIY the CSI 996 query inside deepagents-cli, but it'd duplicate work that belongs upstream.

The hand-off works out neatly: Apple Terminal, iTerm2, WezTerm don't speak CSI 996 even after #6151 ships, so the TERM_PROGRAM mapping doubles as the fallback for those users.

One call: one PR or two (mapping first, system-detect once #6151 lands)?