RCO Editor w/ 6.60 rco files - "Not a valid RCO!"

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I've been trying forever now to make a custom, fancy-shmasncy FF8 ctf theme for my PSP Go. Though I'm sure by now everyone knows the theme modding scene is essentially dead and it's giving me hell trying to find anything.
I'm at the point where I got 6.60 themes to work for 6.61, but I can't edit any of the extracted RCO files from a 6.60 theme or original 61 RCO file with RCOEditor. It just tells me it's "Not a valid RCO!" or the file isn't decompressed respectively.
May someone point me to decompressed 6.61 RCOs, or let me know how to edit the RCOs I have now? I can't seem to find a way to decompress them myself.

submitted by /u/HellBented
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