added color/ theme settings, new config window

backend/theme/config.go

@@ -0,0 +1,80 @@
+// Package theme manages visual theme configuration.
+package theme
+
+import (
+	"errors"
+	"fmt"
+	"regexp"
+)
+
+var (
+	errInvalidHexColor        = errors.New("invalid hex color")
+	errUnknownBackgroundShade = errors.New("unknown background shade")
+)
+
+// hexColorRe matches 3- or 6-digit CSS hex colours (e.g. "#fff", "#ffd43b").
+var hexColorRe = regexp.MustCompile(`^#(?:[0-9a-fA-F]{3}|[0-9a-fA-F]{6})$`)
+
+// BackgroundShade controls the base grayscale palette.
+type BackgroundShade string
+
+// Valid BackgroundShade values.
+const (
+	// BackgroundDarker is an OLED-friendly palette with true black.
+	BackgroundDarker BackgroundShade = "darker"
+
+	// BackgroundDark is the default dark palette.
+	BackgroundDark BackgroundShade = "dark"
+
+	// BackgroundLight is a light-mode palette.
+	BackgroundLight BackgroundShade = "light"
+)
+
+// Defaults.
+const (
+	DefaultAccentColor     = "#ffd43b"
+	DefaultBackgroundShade = BackgroundDark
+)
+
+// Config holds visual theme preferences.
+type Config struct {
+	AccentColor     string          `toml:"AccentColor"`
+	BackgroundShade BackgroundShade `toml:"BackgroundShade"`
+}
+
+// ApplyDefaults fills zero-value fields with sensible defaults.
+func (c *Config) ApplyDefaults() {
+	if c.AccentColor == "" {
+		c.AccentColor = DefaultAccentColor
+	}
+
+	if c.BackgroundShade == "" {
+		c.BackgroundShade = DefaultBackgroundShade
+	}
+}
+
+// Validate checks that all values are well-formed.
+func (c *Config) Validate() error {
+	c.ApplyDefaults()
+
+	if !hexColorRe.MatchString(c.AccentColor) {
+		return fmt.Errorf(
+			"%w: %q",
+			errInvalidHexColor,
+			c.AccentColor,
+		)
+	}
+
+	switch c.BackgroundShade {
+	case BackgroundDarker, BackgroundDark, BackgroundLight:
+		// Valid.
+	default:
+		return fmt.Errorf(
+			"%w: %q",
+			errUnknownBackgroundShade,
+			c.BackgroundShade,
+		)
+	}
+
+	return nil
+}