Martyn,
The 6.0 themes are OK but there are unfortunately quite a few bugs support hasn´t addressed yet but I am hoping they will. I have posted bug reports for all of these over the last couple of weeks and looking at these before deciding which theme to use might be good.
Regarding implementing a AIM 5.9 theme in AIM 6.0, I would not recommend you do that because AIM 6.0 use extjs 5 which has newly designed components that just aren´t available in extjs 4 themes. I would instead recommend you focus on doing the changes you need yourself like changing the size of the tree component etc..
I read RLJB´s post about creating a extjs theme from scratch (using Ruby, Compass etc.) and that is 1 option BUT there is a much simpler option and that is basically using one of the existing themes and build on it (change CSS theme files). I do this for all my applications and you can basically change all aspects of an app ui. The Sencha way, as mentioned by RLJB, is of course the better option in the long run but doing changes yourself directly in the CSS files basically have the same result and works just as good. I will be doing a workshop on this on the upcoming conference and will share my presentation after the event (where I go through this process in detail i.e. step by step).
