Are you using 3.1 Rom? GAAE is based on Workbench 2.05 for 2.0 Rom.
For 3.1 Rom you should use Classic Workbench 68k as it's built in OS3.1
There is no Green Alien theme for 68k but you can use the awesome 1.3 theme:
http://classicwb.abime.net/classicweb/68k.htm
