HQ Customization items can change the appearance of your base. Their effect is temporary and expires after a certain time. You can check how much time remains until your active customization item expires or you can activate a new one by going to HQ > Customize.
Available HQ Customization Items
Empire Skytower - The view from the top floor is second to none
Eiffel Tower - Was build in 1889, so handle with care
Saint Basil's Cathedral - The top dome shines brighter than the brightest star
Desert Palace - Elegance in perfect symmetry
Note that activating an HQ Customization item while another one is active will override the remaining time of the previous customization item.