''<p>delphine.delauzon [[#question-1315468|said]]</p>'' <blockquote>good morning, I downloaded Thunderbird 78, I wanted to install a new theme: I did it from the add-on manager tab, it loads, it says that the theme is enabled, but in fact it's not, it doesn't change. when i close and relaunch thunderbird, i see within one or two seconds the theme that appears and disappears. what should i do? thank you for your help. attached is an example where you can see both the theme that is supposed to be activated and the menu bar that is "off theme". </blockquote> I don't think there's anything wrong with the theme, as it works here and is maintained after restart (see picture). Theme switching in TB 78 is flawed, as you sometimes have to cycle through the Default and Light themes to make a custom theme be applied. The W10 Colors settings can also have an effect. If switching between themes doesn't work, start in safe mode (hold Shift when launching), check 'Reset toolbars & controls', 'Make changes and restart', then see if the theme can be enabled in normal mode.