That does not look like the default theme. Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender.

As Themes are just another add-on. this applies to them as well.

Matt said

That does not look like the default theme. Restart Thunderbird with add-ons disabled (Thunderbird Safe Mode). On the Help menu, click on "Restart with Add-ons Disabled". If Thunderbird works like normal, there is an Add-on or Theme interfering with normal operations. You will need to re-enable add-ons one at a time until you locate the offender. As Themes are just another add-on. this applies to them as well.

This is the default theme, I'll never change it. I have only one addon named "lightning" (its scheduler and calendar) and this is default add on. I have no plugins. After restart thunderbird buttons apear again, but until the next hibernation. thunderbird 52.2.1 x86 russian