client/systray: support several different color themes

Currently we only have a dark theme icon with white and grey dots over
a black background. For some desktops, a logo with black and grey dots
over a white background might be preferable. And for desktops where the
bar is *almost* black or white, but not quite, an option to render the
logo with dots only and no background can look really nice.

Add a new -theme flag to the systray command with the default staying
the same as it is today.

Updates #18303

Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>

client/systray/logo.go

@@ -11,6 +11,7 @@ import (
 	"image"
 	"image/color"
 	"image/png"
+	"log"
 	"runtime"
 	"sync"
 	"time"
@@ -204,12 +205,49 @@ var (
 )
 
 var (
-	bg   = color.NRGBA{0, 0, 0, 255}
-	fg   = color.NRGBA{255, 255, 255, 255}
-	gray = color.NRGBA{255, 255, 255, 102}
-	red  = color.NRGBA{229, 111, 74, 255}
+	black       = color.NRGBA{0, 0, 0, 255}
+	white       = color.NRGBA{255, 255, 255, 255}
+	darkGray    = color.NRGBA{102, 102, 102, 255}
+	lightGray   = color.NRGBA{153, 153, 153, 255}
+	red         = color.NRGBA{229, 111, 74, 255}
+	transparent = color.NRGBA{}
+
+	// default values to dark theme
+	bg   = black
+	fg   = white
+	gray = darkGray
 )
 
+// SetTheme sets the color theme of the systray icon.
+//
+// Supported themes are:
+//   - dark       - white and gray dots over black background
+//   - dark:nobg  - white and grey dots over transparent background
+//   - light      - black and gray dots over white background
+//   - light:nobg - black and grey dots over transparent background
+func SetTheme(theme string) {
+	switch theme {
+	case "dark":
+		bg = black
+		fg = white
+		gray = darkGray
+	case "dark:nobg":
+		bg = transparent
+		fg = white
+		gray = darkGray
+	case "light":
+		bg = white
+		fg = black
+		gray = lightGray
+	case "light:nobg":
+		bg = transparent
+		fg = black
+		gray = lightGray
+	default:
+		log.Printf("unknown theme: %q", theme)
+	}
+}
+
 // render returns a PNG image of the logo.
 func (logo tsLogo) render() *bytes.Buffer {
 	const borderUnits = 1