use termenv HasDarkBackground() to determine default

main.go

@@ -12,7 +12,7 @@ import (
 )
 
 var (
-	term = termenv.ColorProfile()
+	term  = termenv.ColorProfile()
 	theme Theme
 
 	all         = flag.Bool("all", false, "include pseudo, duplicate, inaccessible file systems")
@@ -24,13 +24,13 @@ var (
 	hideBinds   = flag.Bool("hide-binds", true, "hide bind mounts")
 	hideFs      = flag.String("hide-fs", "", "hide specific filesystems, separated with commas")
 
-	output 		= flag.String("output", "", "output fields: "+strings.Join(columnIDs(), ", "))
-	sortBy 		= flag.String("sort", "mountpoint", "sort output by: "+strings.Join(columnIDs(), ", "))
-	width  		= flag.Uint("width", 0, "max output width")
-	themeOpt  	= flag.String("theme", "dark", "color themes: dark, light") 
+	output   = flag.String("output", "", "output fields: "+strings.Join(columnIDs(), ", "))
+	sortBy   = flag.String("sort", "mountpoint", "sort output by: "+strings.Join(columnIDs(), ", "))
+	width    = flag.Uint("width", 0, "max output width")
+	themeOpt = flag.String("theme", "", "color themes: dark, light")
 
-	inodes     	= flag.Bool("inodes", false, "list inode information instead of block usage")
-	jsonOutput 	= flag.Bool("json", false, "output all devices in JSON format")
+	inodes     = flag.Bool("inodes", false, "list inode information instead of block usage")
+	jsonOutput = flag.Bool("json", false, "output all devices in JSON format")
 )
 
 // renderTables renders all tables.
@@ -146,7 +146,10 @@ func parseHideFs(hideFs string) map[string]struct{} {
 func main() {
 	flag.Parse()
 
-	theme = loadThemes(*themeOpt)
+	if *themeOpt == "" {
+		*themeOpt = getDefaultThemeName()
+	}
+	theme = loadTheme(*themeOpt)
 	// validate flags
 	columns, err := parseColumns(*output)
 	if err != nil {

themes.go

@@ -14,7 +14,14 @@ type Theme struct {
 	colorCyan    termenv.Color
 }
 
-func loadThemes(theme string) Theme {
+func getDefaultThemeName() string {
+	if !termenv.HasDarkBackground() {
+		return "light"
+	}
+	return "dark"
+}
+
+func loadTheme(theme string) Theme {
 	themes := make(map[string]Theme)
 
 	themes["dark"] = Theme{