You can use the /loadui command, but while it "works," it can play havoc with certain UI elements (mostly fonts and some dynamic data).
The syntax is
/loadui youruiname where "youruiname" is your custom UI's exact folder name, including capitalization; i.e.,
/loadui customui or
/loadui Default
It's an ok method to use when you're doing a lot of UI testing, but once you've got everything in place, you should reboot your client so that everything looks and works as it should.
