Exported themes cannot be used in Firefox. Attempting to import them produces an error message.

Export function outdated, won't work for Addons Marketplace

The .xpi file still uses manifest v2, which is no longer accepted. This causes errors like “incomplete theme”. Mozilla hasn't updated this export feature even though v3 has been required for a while.

A workaround for sidebar text contrast issues when using dark theme.
:root {
color-scheme: light dark !important;
}