As of May 22, the system for theme management has changed on the repository. A fix has been implemenmted, but if another change arises and causes this app to work, please consider opening an issue.
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Theme Store app by Sebastian Maung
https://github.com/SebastianMaung/MiyooThemeDownloader
Built for Onion OS
https://github.com/OnionUI/Onion

Themes are pulled from here:
https://github.com/OnionUI/Themes


After moving this app to the App directory of the Miyoo, launch it (Onion OS Theme Store)
It will start downloading the previews, which may take up to 5 minutes
If at any point you want to stop the downloads, press X, then menu (canceling it might result in some themes not showing)
It will take you to a UI with all the previews of the themes
If you are downloading a theme, press A on the theme you want. If you want to download an icon pack, press X.
You will then be sent to the theme switcher, where you can choose your newly downloaded theme
If you do not want to change anything, press menu

If the app isn't working, please open up an issue. I would hate to be creating a pulbic app that didn't work as intended.