Theme Skills are skills that are specific to individual Sync Pairs but can be used in tandem with other Sync Pairs for even stronger teams. These Theme Skills could range from the type of Sync Pair, the region they come from, and more to provide rewards like extra HP for that particular Sync Pair.
Every Sync Pair has up to five different Theme Skills that can be upgraded using Skill Spheres. Each upgrade requires a different amount of Skill Spheres, so it would be best to first acquire as many as you can before upgrading your Theme Skills.
To get to the window where you can upgrade your Theme Skills, you will want to begin by selecting the Sync Pairs option at the bottom of the game's main home screen. You will then want to select the Theme Skills option from this menu.
You will then be provided with all of the Sync Pairs available for you to upgrade theme skills. The Sync Pairs you have already upgraded to the maximum amount will be at the top of this list and will not have a blue arrow above the image.
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