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_test.go

@@ -0,0 +1,39 @@
+package root
+
+import (
+	"os"
+	"strings"
+	"testing"
+
+	"github.com/versenilvis/iris/internal/config"
+)
+
+func TestThemeInitCommand(t *testing.T) {
+	tmpDir, err := os.MkdirTemp("", "iris-theme-cmd-test")
+	if err != nil {
+		t.Fatalf("failed to create temp dir: %v", err)
+	}
+	defer os.RemoveAll(tmpDir)
+
+	// UserConfigDir follows HOME on macOS and XDG_CONFIG_HOME on Unix.
+	// Override both so the command cannot read or write the user's real theme.
+	t.Setenv("HOME", tmpDir)
+	t.Setenv("XDG_CONFIG_HOME", tmpDir)
+
+	ThemeInitCmd.Run(ThemeInitCmd, []string{})
+
+	themePath, err := config.ThemePath()
+	if err != nil {
+		t.Fatalf("failed to get theme path: %v", err)
+	}
+	if _, statErr := os.Stat(themePath); statErr != nil {
+		t.Errorf("expected theme file to be created at %s, but it was not", themePath)
+	}
+	content, err := os.ReadFile(themePath)
+	if err != nil {
+		t.Fatalf("failed to read theme file: %v", err)
+	}
+	if !strings.Contains(string(content), "sel_text = \"#110f18\"") {
+		t.Error("expected initialized theme to include sel_text = \"#110f18\"")
+	}
+}