[Tails-ux] Thunderbird column dividers missing

Tails 4.1.

I noticed recently that there are no visible column dividers in the
column headers in the main messages viewer window in Thunderbird. This
makes it impossible to know where to click to resize the columns, you
have to move the cursor across the headers until the cursor changes to
the Resize cursor.

Here's a screenshot:

https://cloud.disroot.org/apps/gallery/s/dY4zzG75oXzPPN5