In the Australis menupanel and in the customisation screen there are many wierd stylings going on. Reversing those styling to be more normal and more fitting with your own themes is difficult but can be done.
A very particular thing is that the Customisation mode only works if the #tab-view-deck gets some padding transformation so it can trigger on the end of the transformation. The resizing of everything is very disturbing (in many ways) just for the customisation modus to start.
I have addressed by changing the padding change to 1px and adding a margin of -1px and a transition of 1ms, so that the animation is invisible but still trigger. Possible using another transition-property will also work.
Also in the menupanel, toolbarbuttons are used as menuitems, so I needed to override those styles in order make these more normal again.
Take a look at my themes such as Nautipolis and Walnut on how I did this.
P.s. my themes are also updated for Seamonkey (including Walnut2 as a 'new' theme for Seamonkey). Possibly I will also republish MicroMonkey again.