Thanks for the quick reply - I have tried using the 'dark one' theme but there is no change, although thanks to your suggestion of outputting to a window I think I can now see where the problem lies, but solving it is another matter...
21:02:03.302: QMC2 Arcade 0.34 (Qt 4.8.7, emulator-mode: mame, console-mode: window-minimized, graphics-system: native, language: us, theme: darkone)
21:02:03.311: SDL joystick #0 opened: name = USB,2-axis 8-button gamepad , axes = 2, buttons = 8, hats = 0, trackballs = 0
21:02:03.311: Loading configuration template from '/Library/Application Support/qmc2/opt/SDLMAME/template-SDL2.xml'
21:02:03.317: Done (loading configuration template from '/Library/Application Support/qmc2/opt/SDLMAME/template-SDL2.xml') - 260 options loaded
21:02:03.317: WARNING: Can't open cabinet 7z file '/Applications/qmc2/mame/cabinets.zip'
21:02:06.401: DAT-info database: importing emulator info-texts from '/Applications/qmc2/mame/mameinfo.dat'
21:02:07.899: DAT-info database: done (importing emulator info-texts from '/Applications/qmc2/mame/mameinfo.dat')
21:02:07.900: DAT-info database: importing emulator info-texts from '/Applications/qmc2/mame/messinfo.dat'
21:02:08.388: DAT-info database: done (importing emulator info-texts from '/Applications/qmc2/mame/messinfo.dat')
21:02:08.389: DAT-info database: 12885 emulator info records imported
21:02:08.395: Starting QML viewer using theme 'darkone'
21:02:08.409: QtWarningMsg: qrc:/qml/darkone/1.1/darkone-video.qml:2:1: module "QtMultimediaKit" is not installed
21:02:08.510: Activating windowed display
21:02:10.741: Stopping QML viewer
I have tried reinstalling the SDL2-2.0.4 framework in the Library folder with no effect.