feat(config): add theme customization (#81)

Added theme customization via config.toml
#17
```
[theme]
border = "#665C54"
accent = "#FABD2F"
muted = "#928374"
text = "#EBDBB2"
text_sel = "#FBF1C7"
match = "#FE8019"
desc = "#A89984"
desc_sel = "#EBDBB2"
sel_bg = "#504945"
scroll_info = "#83A598"
ghost_text = "#7C6F64"
hist= "#fe8019"
hist_sel = "#d65d0e"
sys = "#fb4934"
sys_sel = "#cc241d"
alias = "#b8bb26"
alias_sel = "#98971a"


```

---------

Co-authored-by: verse91 <versedev.store@proton.me>
Co-authored-by: VERSE <sunnyone0901@gmail.com>

root/theme_cmd.go

@@ -0,0 +1,106 @@
+package root
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/spf13/cobra"
+	"github.com/versenilvis/iris/internal/config"
+)
+
+var ThemeCmd = &cobra.Command{
+	Use:   "theme",
+	Short: "manage iris theme",
+}
+
+var ThemeInitCmd = &cobra.Command{
+	Use:   "init",
+	Short: "initialize default theme file with comments",
+	Run: func(cmd *cobra.Command, args []string) {
+		path, err := config.ThemePath()
+		if err != nil {
+			fmt.Printf("failed to get theme path: %v\n", err)
+			return
+		}
+
+		if _, statErr := os.Stat(path); statErr == nil {
+			fmt.Printf("theme file already exists at %s\n", path)
+			return
+		}
+
+		_ = os.MkdirAll(filepath.Dir(path), 0755)
+
+		defaultContent := `# ~/.config/iris/theme.toml
+# iris theme file
+
+# overlay border color
+border = "#a277ff"
+
+# accent color for selected marker and icon
+accent = "#61ffca"
+
+# muted color for unselected icons
+muted = "#6d6a7f"
+
+# default text color
+text = "#edecee"
+
+# text color when selected
+text_sel = "#ffffff"
+
+# footer shortcut key color
+key = "#a277ff"
+
+# matched text color
+match = "#61ffca"
+
+# description text color
+desc = "#9692a8"
+
+# description text color when selected
+desc_sel = "#edecee"
+
+# selected row background
+sel_bg = "#3d375e"
+
+# selected tag pill text color
+sel_text = "#110f18"
+
+# scroll counter color
+scroll_info = "#a277ff"
+
+# inline ghost text color
+ghost_text = "#4B4A4C"
+
+# "system" tag background color
+sys = "#1e1d28"
+
+# "system" tag selected background color
+sys_sel = "#a277ff"
+
+# "history" tag background color
+hist = "#1a2d36"
+
+# "history" tag selected background color
+hist_sel = "#61ffca"
+
+# "alias" tag background color
+alias = "#2a2342"
+
+# "alias" tag selected background color (and alias word color)
+alias_sel = "#a277ff"
+`
+		err = os.WriteFile(path, []byte(defaultContent), 0644)
+		if err != nil {
+			fmt.Printf("failed to write theme file: %v\n", err)
+			return
+		}
+		fmt.Printf("initialized theme file at %s\n", path)
+	},
+}
+
+func init() {
+	ThemeCmd.AddCommand(ThemeInitCmd)
+	rootCmd.AddCommand(ThemeCmd)
+}