feat: Add Dracula and Everforest color themes, set Everforest as default

- Add DraculaTheme with official Dracula color palette
- Add EverforestTheme with natural Everforest dark color scheme
- Set Everforest as default theme (was 'default')
- Update README with theming documentation and THEME env var usage
- Build verified successfully

internal/tui/theme/theme.go

@@ -79,11 +79,39 @@ var LightTheme = &Theme{
 	},
 }
 
+// DraculaTheme is the popular Dracula dark color scheme
+var DraculaTheme = &Theme{
+	Name: "dracula",
+	Scheme: ColorScheme{
+		Primary:    lipgloss.Color("#BD93F9"), // Purple
+		Success:    lipgloss.Color("#50FA7B"), // Green
+		Warning:    lipgloss.Color("#FFB86C"), // Orange
+		Error:      lipgloss.Color("#FF5555"), // Red
+		Muted:      lipgloss.Color("#6272A4"), // Light purple-gray
+		Background: lipgloss.Color("#282A36"), // Dark background
+	},
+}
+
+// EverforestTheme is the natural Everforest dark color scheme
+var EverforestTheme = &Theme{
+	Name: "everforest",
+	Scheme: ColorScheme{
+		Primary:    lipgloss.Color("#7fbbb3"), // Blue
+		Success:    lipgloss.Color("#a7c080"), // Green
+		Warning:    lipgloss.Color("#dbbc7f"), // Yellow
+		Error:      lipgloss.Color("#e67e80"), // Red
+		Muted:      lipgloss.Color("#414b50"), // Gray
+		Background: lipgloss.Color("#272e33"), // Dark background
+	},
+}
+
 // ThemeRegistry holds all available themes
 var ThemeRegistry = map[string]*Theme{
-	"default": DefaultTheme,
-	"dark":    DarkTheme,
-	"light":   LightTheme,
+	"default":    DefaultTheme,
+	"dark":       DarkTheme,
+	"light":      LightTheme,
+	"dracula":    DraculaTheme,
+	"everforest": EverforestTheme,
 }
 
 // GetTheme loads the theme from config or environment variable
@@ -93,15 +121,15 @@ func GetTheme() (*Theme, error) {
 		// Try to get theme from environment variable first
 		themeName := os.Getenv("THEME")
 		if themeName == "" {
-			themeName = "default"
+			themeName = "everforest"
 		}
 
 		// Look up the theme in the registry
 		if theme, ok := ThemeRegistry[themeName]; ok {
 			currentTheme = theme
 		} else {
-			// If theme not found, use default
-			currentTheme = DefaultTheme
+			// If theme not found, use everforest as default
+			currentTheme = EverforestTheme
 		}
 
 		// Apply the theme to initialize styles