In Ubuntu, the ImageCashe.png file is placed in the Theme subfolder to be created in the .audacity-data folder located in the home directory (hidden files should be visible - Ctrl+H). In the Audacity Preferences section of the Interface tab, the “Custom” option must be selected in the Theme box. You can experiment how it looks better by activating and deactivating the “Blend system and Audacity theme” option. Have fun!