Rio dev here. You're right in that all Rio components use Material Design, but there is still room for configuration. I can't post pictures here, but if you check our discord, somebody has made a full-retro website that doesn't look material-y at all.
In addition to high-level components there's also some very basic building blocks, like a humble Rectangle. By combining these in clever ways you can get more styles than you might originally expect - just like everything on the web is ultimately colored boxes.
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