How do I put other themes on?

[jxx2005]

I refuse to update to 3.90-3 because my PS1 games work best on 3.71-4 (in fact I know several people who say 3.71-4 is the best CFW out there to have).

thats what the popsloader is for
it lets you run PS1 games using the files from 3.71 (or whatever other firmware you want)

[Ultimate warrior]

Yes but I tried to figure popsloader out once and failed miserably.