A1)
It's my bug. I'll fix it.
If you change externals more radically,
you can use skin feature.
copyall images/default/ to another name
set the directory name into preferences of piCal
change it
A2)
The popup Calendar is not a part of piCal, but a part of XOOPS core.
If you'd like to change it, claim it to
www.xoops.org or dutch xoops site.