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I am on 3.52 m33-4 and themes work great. You just have to look in old threads at forums for some themes.
3.52 M33 is still very popular because it is the last 1.50 kernel based hybrid firmware ( it uses a modified 1.50 IPL+ 1.50 kernel as the KD folder in flash0)
My PSP Stats:
Model: An American PSP-1001 /TA-082 "Fat"/ 01g
My Firmwares:
3.52 M33-4 (NAND)
1.50( MS)
3.60 M33( MS, but for backup reasons and future use*)
4.01 M33-2 (MS, but very useful)