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>

internal/config/paths.go

@@ -17,6 +17,18 @@ func ConfigPath() (string, error) {
 	return filepath.Join(dir, "iris", "config.toml"), nil
 }
 
+func ThemePath() (string, error) {
+	configHome := os.Getenv("XDG_CONFIG_HOME")
+	if configHome != "" {
+		return filepath.Join(configHome, "iris", "theme.toml"), nil
+	}
+	dir, err := os.UserConfigDir()
+	if err != nil {
+		return "", err
+	}
+	return filepath.Join(dir, "iris", "theme.toml"), nil
+}
+
 func StatePath() (string, error) {
 	dataHome := os.Getenv("XDG_DATA_HOME")
 	if dataHome == "" {