If you create a custom appearance for a widget, and want this to always be used by default, put lines like this in your module file, somewhere after the GXT inherits statements:
This will cause the compiler to automatically always use your theme instead of the defaults provided. If you have a library of custom themes or widgets used internally, you might make a whole set of appearances, as are in com.sencha.gxt.theme.base (the basics for making the standard GXT themes) and com.sencha.gxt.theme.blue (specific details for making the images and colors blue). It isn't required to inherit Base or Blue, but may be handy for some initial dom structure that already works.
<replace-with class="my.package.client.theme.MyFieldLabelAppearance"> <when-type-is class="com.sencha.gxt.widget.core.client.form.FieldLabel.FieldLabelAppearance" />
If you only want some instances to use, or want some to be different than others, then either your css could have some extra class name to switch between the two, or make one the default (with the replace-with rebind rule), and pass the other appearance into the others' constructor.