Did you at least bother trying to research first, even if only by looking at their official website?
Make 12 folders (maximum - represents the months of the year) named theme01, theme02, and so on, and place them within the _system_ folder on your microsd.
Download ready-made skins from this forum/elsewhere, or make your own and place them within those individual folders.
The following I have *snipped* from the R4DS official website:
Switch of the Skins
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If the Skin exists in the directory of _system_ in microSD, it automatically enters multi Skins mode.
The Skins in multi Skins mode can be switched manually and automatically, it is automatically switched with the Skins by months.
The method of the Skin switch is: Click “SELECT” key in main menu, it switches between twelve manual modes and one auto mode.