mail: linked thunderbird

wiki/mail.md

@@ -22,3 +22,6 @@ And even if you have it can be useful to keep your mails synced locally on your
 
 - [Neomutt](linux/neomutt.md) is a [terminal-based](/wiki/system_console.md)
   mail client for linux, that has a simple interface.
+- [Thunderbird](/wiki/thunderbird.md) is a free and open source mail client which is cross
+  platform.
+  It also features, contact, todo and calendar management.

wiki/thunderbird.md

@@ -0,0 +1,16 @@
+# Thunderbird
+
+[Thunderbird](https://www.thunderbird.net/en-US/) is a free and open-source program to manage
+numerous e-mail addresses, calendars, todos and contacts.
+
+## Usage
+
+The following section addresses different uses and add-ons of Thunderbird.
+
+### Changing the (dark/light) Theme
+
+This section is based on a
+[superuser comment by blnks](https://superuser.com/questions/1757333/how-can-i-view-thunderbird-in-full-dark-mode).
+The dark theme can not be enabled from the default preferenes or settings.
+To turn on the dark theme navigate to `Tools` and `Add-ons and Themes`.
+There select `Enable` for the theme of your choice.