You are using a dark theme. In Thunderbird check to see what theme you are using. Menu app icon > Addons > Addons > Themes For the 'Light' theme click on 'Enable' button.

Please note that Windows 10 also offers Light and Dark mode for apps. As a heads up, in future it would be helpful if you could tell the forum what version you are using when posting a question. I'm presuming you are using a version of 68*. This is because the instructions are different for those using 78*.

Many thanks. Yes, I am using Thunderbird 68, though it is kept up to date. Anyway, I have got rid of the unwanted black panes. Grateful for your help.

I am running 78.5.1 on a Win7 system. Just since the update to 78 I now have panes that do not display automatically, they are black until I move over them with the mouse. I am have not changed the default color scheme, so it's not a dark theme issue. I have a separate nvidia graphics card in my Lenovo laptop that the troubleshooting info shows are being used instead of the other built-in Intel graphics. If I close and reopen TB, the screens are good for a few minutes. Then it gets to where it slow to load, e.g. click on the slider bar in the nav pane (I have several accounts and use lots of folders / subfolders) and there's a discernible delay where the pane is black before updating. After a few more minutes it just stops loading panes at all until I mouse over. At the same time, other panes will stay light but fail to update as well, for example the list of emails in the subfolder. Very annoying, I may go back to 68 if this persists. Thanks for any help / fixes!