The 'Light' or 'Dark' mode on a computer is very specific and it's a 'Theme' which is why it is located under 'Addons and Themes'. It has nothing to do with 'Language' and 'Appearance' is about font and colour and size - it has nothing to do with Themes. Please note it is possible to download a Theme, so that means it's an addon hence why they are in same location. It's not going to change because a generic Theme is not the same as choosing how you prefer to alter the appearance of the contents when using the selected theme. '''re :but can't find a solution to switch to a readable light version. ''' I suggest you offer more information on your real issue and see if a good solution can be found that suits you. Please tell me what OS you are using because the 'System Theme' on Thunderbird will be dependent upon what settings you have on computer. For example: Windows OS has a settings for customised app mode which is dark or light. Windows OS - Settings > Personalisation > Colours will also effect eg: Task bar, button background colours and the thin strip colour which depicts what eg: Quick Filter optins are set. So if computer has apps mode set to Light then the 'System Theme' setting in Thunderbird will also be Light. Please offer info about what you are finding difficult to 'read' and perhaps post an image to help explain the problem.