What causes "unknown theme: %s" in muesli/duf?

Run with one of the supported theme names: dark, light, or ansi

unknown theme: %s

Error message

unknown theme: %s

What it means

loadTheme builds a fixed map of supported themes ("dark", "light", "ansi") and returns this error when the requested theme name is not a key in that map. It is a simple lookup failure: the theme string passed in (typically from a CLI flag or config) does not match any registered theme. The error message includes the offending theme name so the caller can see what was rejected.

Source

Thrown at themes.go:74

		colorBgYellow: env.Color("#fff4d0"),
		colorBgGreen:  env.Color("#e6ffe6"),
	}

	themes["ansi"] = Theme{
		colorRed:      env.Color("9"),
		colorYellow:   env.Color("11"),
		colorGreen:    env.Color("10"),
		colorBlue:     env.Color("12"),
		colorGray:     env.Color("7"),
		colorMagenta:  env.Color("13"),
		colorCyan:     env.Color("8"),
		colorBgRed:    env.Color("1"),
		colorBgYellow: env.Color("3"),
		colorBgGreen:  env.Color("2"),
	}

	if _, ok := themes[theme]; !ok {
		return Theme{}, fmt.Errorf("unknown theme: %s", theme)
	}

	return themes[theme], nil
}

View on GitHub (pinned to 4636deb4a7)

Solutions

Run with one of the supported theme names: dark, light, or ansi
Check the exact spelling and casing — the lookup is case-sensitive, so "Dark" fails
If the theme came from a config file or env var, fix or remove that value
List available themes from the tool's help output (or themes map in themes.go) and pick a valid one
If a new theme is genuinely needed, add it to the themes map in loadTheme (or contribute it upstream)

Example fix

// before
loadTheme("Dark") // -> unknown theme: Dark
// after
loadTheme("dark") // valid: dark | light | ansi

Defensive patterns

Strategy: validation

Validate before calling

var validThemes = []string{"dark", "light", "ansi"}
func isValidTheme(name string) bool {
	for _, t := range validThemes {
		if t == name {
			return true
		}
	}
	return false
}
// before calling: if !isValidTheme(themeName) { themeName = "dark" }

Type guard

func knownTheme(name string) (string, bool) {
	switch name {
	case "dark", "light", "ansi":
		return name, true
	}
	return "", false
}

Try / catch

theme, err := loadTheme(name)
if err != nil {
	fmt.Fprintf(os.Stderr, "warning: %v; falling back to default theme %q\n", err, defaultThemeName())
	theme, err = loadTheme(defaultThemeName())
	if err != nil {
		fmt.Fprintln(os.Stderr, "fatal: default theme failed:", err)
		os.Exit(1)
	}
}

Prevention

Validate the theme flag in CLI arg parsing before invoking loadTheme
Default to defaultThemeName() when the flag is empty or unset
Normalize input (strings.ToLower, strings.TrimSpace) and compare against the whitelist
Keep the accepted theme list documented in --help so users don't guess names
Test config-driven theme values with a fallback path in mind

When it happens

Trigger: Calling loadTheme(name) where name is not exactly "dark", "light", or "ansi" — e.g. loadTheme("solarized"), loadTheme("Dark") (case mismatch), loadTheme(""), or a misspelled name like "dakr". main reaches this whenever a user supplies an unlisted -theme/--theme flag value or a config file sets theme to an unknown name.

Common situations: A user passes --theme=monokai expecting extra built-in themes; a config file written for another tool carries a theme name this tool doesn't support; a typo or wrong casing ("Dark" vs "dark"); an empty theme value from an unset env var or blank CLI flag; version drift where a theme name was removed/renamed.

Understand the failure class

Background: Invalid enum value errors: "Unknown type", "Invalid scope", "must be one of" — when a string is not on the library's allowed list — this error's family across 23 libraries.

Related errors

AI-assisted analysis of muesli/duf@4636deb4a7 (2026-09-06). Data as JSON: /api/errors/aea2babf10f7b3cc. Report an issue: GitHub.