theme: prevent failed portal writes from reverting theme mode

related: #2786

port 1.5

core/internal/matugen/matugen.go

@@ -260,15 +260,15 @@ func Run(opts Options) error {
 		return buildErr
 	}
 
+	if opts.SyncModeWithPortal {
+		syncColorScheme(opts.Mode)
+	}
+
 	if !changed {
 		log.Info("No color changes detected, skipping refresh")
 		return ErrNoChanges
 	}
 
-	if opts.SyncModeWithPortal {
-		syncColorScheme(opts.Mode)
-	}
-
 	log.Info("Done")
 	return nil
 }
@@ -1006,6 +1006,13 @@ func syncColorScheme(mode ColorMode) {
 		scheme = "default"
 	}
 
+	if cur, err := utils.GsettingsGet("org.gnome.desktop.interface", "color-scheme"); err == nil {
+		cur = strings.Trim(cur, "'")
+		if cur == scheme || (mode == ColorModeLight && cur == "prefer-light") {
+			return
+		}
+	}
+
 	if err := utils.GsettingsSet("org.gnome.desktop.interface", "color-scheme", scheme); err != nil {
 		log.Warnf("Failed to sync color-scheme: %v", err)
 	}