You can manage your store’s design themes using the Themes page of your store's Admin area (Store Design > Themes).
At the top of the Themes page, you can see the design theme that is active (enabled) in the store currently.
The default design theme for X-Cart 5.5.x is Crisp White. Your store comes with this theme installed and enabled.
Your store's Admin area provides UIs that you can use to adjust this theme or to switch to a different theme.