Lazily detect background color setting

This change prevents Lipgloss from querying the terminal's background
color setting on init. Instead, the background color will get queried
the first time the setting gets accessed by HasDarkBackground, and the
result will be cached.

Furthermore SetHasDarkBackground(bool) has been added, which explicitly
sets a value and avoids the automatic detection from ever being run.

color.go

@@ -11,12 +11,12 @@ var (
 	colorProfile         termenv.Profile
 	getColorProfile      sync.Once
 	explicitColorProfile bool
-	colorProfileMtx      sync.Mutex
 
-	// Because it's a potentially long operation (relatively speaking), we
-	// check the background color on initialization rather than at the last
-	// possible second.
-	hasDarkBackground = termenv.HasDarkBackground()
+	hasDarkBackground       bool
+	getBackgroundColor      sync.Once
+	explicitBackgroundColor bool
+
+	colorProfileMtx sync.Mutex
 )
 
 // ColorProfile returns the detected termenv color profile. It will perform the
@@ -53,11 +53,34 @@ func SetColorProfile(p termenv.Profile) {
 	explicitColorProfile = true
 }
 
-// HadDarkBackground returns whether or not the terminal has a dark background.
+// HasDarkBackground returns whether or not the terminal has a dark background.
 func HasDarkBackground() bool {
+	if !explicitBackgroundColor {
+		getBackgroundColor.Do(func() {
+			hasDarkBackground = termenv.HasDarkBackground()
+		})
+	}
+
 	return hasDarkBackground
 }
 
+// SetHasDarkBackground sets the value of the background color detection on a
+// package-wide context. This function exists mostly for testing purposes so
+// that you can assure you're testing against a specific background color
+// setting.
+//
+// Outside of testing you likely won't want to use this function as
+// HasDarkBackground() will detect and cache the terminal's current background
+// color setting.
+//
+// This function is thread-safe.
+func SetHasDarkBackground(b bool) {
+	colorProfileMtx.Lock()
+	defer colorProfileMtx.Unlock()
+	hasDarkBackground = b
+	explicitBackgroundColor = true
+}
+
 // TerminalColor is a color intended to be rendered in the terminal. It
 // satisfies the Go color.Color interface.
 type TerminalColor interface {