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Which theme of dig supports the most systems
That's hard to answer. If you add clones, Unified Arkade has a lot of clones. Given many themes don't say what systems they have exactly, you're not going to get an easy answer.
Yes,you would have to download them yourself,open the ZIP and check the system icon folder.
Easier just to download any themes you like,if any system icon is missing that you use,kindly ask the theme donator to add them.😀
Unified Arkade does state 109 systems for support so that's your best bet if you just want system quantity.