what would happen if i installed a 8700 theme in my 7520? same as installing a 7100 theme in my 7520. what would happen?

It would not be good. You would probably end up have to reload the OS.

They would show up but give you no option to activate them - at best. You would have to re-load the OS - at worst...

Assuming your 7520 has OS 4.1, then it may be possible to load it. The only issue is that the resolution on an 8700 is SO much greater than that of the 7520, thus the theme would NOT fit. The same goes for a 7100 theme. The 7520 default theme would fit on a 7100 and somewhat fit (although adjusted to the left margin) on the 8700.

In other words, there's no point. Its doubtful you'd have to reload the OS, in any case. At worst, you'd have a theme that either did not show up (although the module was present on the handheld) or showed up and only had the option to Close (no Activate option).