removeTheme method
Removes the theme with the given id dynamically.
If this fails(possibly non existing theme id), throws an error.
Implementation
void removeTheme(String themeId) {
if (!hasTheme(themeId)) {
throw Exception('$themeId does not exist.');
}
if (currentThemeId == themeId) {
throw Exception('$themeId is set as current theme.');
}
_appThemes.remove(themeId);
_appThemeIds.remove(themeId);
notifyListeners();
}