refactor: change dark theme color

internal/domain/config.go

@@ -117,6 +117,13 @@ const (
 	LLMProviderOpenAI = "openai"
 )
 
+// Theme preference identifiers stored in AppConfig.Theme.
+const (
+	ThemeAuto  = "auto"
+	ThemeLight = "light"
+	ThemeDark  = "dark"
+)
+
 const DefaultSummaryPrompt = `你是一个截图内容总结助手。请阅读截图，并用中文输出一段适合直接粘贴到聊天、工单、Issue 或 PR 中的说明。
 
 要求：
@@ -141,6 +148,9 @@ type AppConfig struct {
 	// in the infrastructure hotkey adapter.
 	Hotkey string `json:"hotkey"`
 
+	// Theme controls the settings UI appearance: "auto", "light", or "dark".
+	Theme string `json:"theme"`
+
 	// S3 holds the active S3-compatible storage configuration.
 	S3 S3Config `json:"s3"`
 
@@ -156,6 +166,7 @@ type AppConfig struct {
 func DefaultAppConfig() AppConfig {
 	cfg := AppConfig{
 		Hotkey: "cmd+shift+a",
+		Theme:  ThemeAuto,
 		S3: S3Config{
 			PathPrefix:   "snapgo/",
 			UsePathStyle: true,
@@ -214,6 +225,9 @@ func (c *AppConfig) Normalize() {
 	if c.Hotkey == "" {
 		c.Hotkey = "cmd+shift+a"
 	}
+	if c.Theme != ThemeLight && c.Theme != ThemeDark && c.Theme != ThemeAuto {
+		c.Theme = ThemeAuto
+	}
 	if c.S3.PathPrefix == "" {
 		c.S3.PathPrefix = "snapgo/"
 	}