Stop using deprecated theme, improve docs

Fixes #4208

cmd/fyne_demo/main.go

@@ -238,10 +238,10 @@ func makeNav(setTutorial func(tutorial tutorials.Tutorial), loadPrevious bool) f
 
 	themes := container.NewGridWithColumns(2,
 		widget.NewButton("Dark", func() {
-			a.Settings().SetTheme(theme.DarkTheme())
+			a.Settings().SetTheme(&forceVariantTheme{Theme: theme.DefaultTheme(), variant: theme.VariantDark})
 		}),
 		widget.NewButton("Light", func() {
-			a.Settings().SetTheme(theme.LightTheme())
+			a.Settings().SetTheme(&forceVariantTheme{Theme: theme.DefaultTheme(), variant: theme.VariantLight})
 		}),
 	)
 

cmd/fyne_demo/theme.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"image/color"
+
+	"fyne.io/fyne/v2"
+)
+
+type forceVariantTheme struct {
+	fyne.Theme
+
+	variant fyne.ThemeVariant
+}
+
+func (f *forceVariantTheme) Color(name fyne.ThemeColorName, _ fyne.ThemeVariant) color.Color {
+	return f.Theme.Color(name, f.variant)
+}

theme/theme.go

@@ -27,6 +27,7 @@ const (
 // DarkTheme defines the built-in dark theme colors and sizes.
 //
 // Deprecated: This method ignores user preference and should not be used, it will be removed in v3.0.
+// If developers want to ignore user preference for theme variant they can set a custom theme.
 func DarkTheme() fyne.Theme {
 	theme := &builtinTheme{variant: VariantDark}
 
@@ -48,6 +49,7 @@ func DefaultTheme() fyne.Theme {
 // LightTheme defines the built-in light theme colors and sizes.
 //
 // Deprecated: This method ignores user preference and should not be used, it will be removed in v3.0.
+// If developers want to ignore user preference for theme variant they can set a custom theme.
 func LightTheme() fyne.Theme {
 	theme := &builtinTheme{variant: VariantLight}