The simplest may be to use (or edit) one of these ready-made themes, following the instructions given to set audacity.cfg to load the theme:
http://jcsu.jesus.cam.ac.uk/~hdc21/design/audacity/ .
Otherwise (assuming you are using Audacity 1.3.9 or later) Audacity cannot create ImageCache.png or the individual theme files for you to edit because Theming is disabled. You can read about the limited and somewhat buggy Theming feature at
http://manual.audacityteam.org/man/Theme_Preferences .
If you want a current build with Theming enabled, build Audacity from source code, either the source tarball:
http://audacityteam.org/download/source#recdown
or current HEAD:
http://wiki.audacityteam.org/wiki/CompilingAudacityForBeginners#simple
then you can enable #define EXPERIMENTAL_THEME_PREFS in src/Experimental.h by removing the two “//” in front of the define. There is also a #define EXPERIMENTAL_THEMING which sets a specific theme but it’s unknown how that reacts to a dark operating system theme like Ubuntu’s default theme.
Gale