Hi Siri,
Creating another Portfolio Item Type, like Theme, will change the hierarchy, but you can control how the hierarchy is changed. When a new Portfolio Item Type is added it will automatically be at the top. If this is where you want the type to exist then no other changes are needed.
If you needed it to be in the middle then you would need to actually change the name of the existing types in addition to creating the type so the hierarchy is in the order you want. You wouldn't need to actually rearrange the types, or to move items.
An administrator can edit the Portfolio Item Types by going to setup by clicking the Tool icon in the upper right. Select the Workspaces & Projects tab and then locate the desired workspace from the list. Click it to go to its detail page. Select the Fields option from the list on the left and then choose Portfolio Item for the type. Locate the Portfolio Item Type field and click the gear next to its name to edit it. This will bring up an edit window. In this window you can create new item types and change the name of existing ones. It is not possible to rearrange them. Instead you would rename them so they are in the desired order. It is also possible to delete them, but that can be difficult so its important to be sure you want the new item type.